How is anyone figuring that the German empire would have to PAY Belgium anything? If Germany has decisively defeated France, no one can extract reparations from Germany. If Russia fights on, it'll be defeated, too. (At best, France might have to pay for rebuilding Belgium--more likely, Belgium is left to pick up its own pieces, or forced to become a German client state. Of course, Belgium comes with the Congo as a bonus add-on, to be sorted at the peace treaty.
I'd expect a peace based on the current situation with regards to Britain, perhaps with Britain giving some German territories back in order to get less onerous terms on the French--a vital counterbalance to Germany on the continent.
The USA will sell Germany whatever it wants; the sea lanes are open.
The US paid Mexico at the end of the Mexican-American war in exchange for the land ceded to the US, if Germany gets all or part of Belgium Congo I could see payment being sent as part of the negotiation.
For me a decisive victory would be Paris either encircled or captured, which means the costal region to the North would be in German hands with Belgium overrun as well. Combined with one or more French army captured in full along with some of the BEF I do think a negotiated peace with Central Power advantage would be negotiated. Of course the big potential game changers here are when do the Central Powers start doing better on the Western Front? If it is early enough you can have some major changes like neutral (or even CP) Italy, earlier entry of Bulgaria, possible CP Romania (they might want a piece of Russia).
Italy and Bulgaria both joined the war OTL in 1915, Romania in 1916 so Romania has not joined as OP states decisive victory before 1916.
Scenario 1: OTL join dates (Entente Italy and CP Bulgaria)
Situation- A more devastating Autumn offensive for the Entente (more lopsided casualty ratio and higher expenditure of munitions including artillery shells), perhaps some of the offensive forces being captured in counter attacks on original German positions, followed by a German counter attack that punches through a drawn down sector (maybe faulty intelligence reaches the French and British so they expect an attack somewhere else and end up pulling forces out of the actual offensive area), end result an actually breach in the defensive line is generated combined with a lack of allied reinforcements for a counter-offensive until German reinforcements arrive and secure the breach- from there a retreat along the front until the Germans are on the outskirts of Paris. Though can someone with more knowledge of WWI warfare give the effects of a successful breakthrough at any point of the Western Front, and by that I mean the attacking force breaching the various defensive lines and not being thrown back or halted by a counter offensive but actually getting out into the rear areas. I went with the Autumn offensive as that occurs close enough to Bulgaria joining and could be a possible POD- for example the French and British think their attacks are closer to succeeding then they actually are, or are close to breaching the German line and so throw more forces into the grinder in the hopes of victory only for German reinforcements to arrive in time and inflict heavy damage on the offensive, that could cause even more expenditure of artillery munitions by the French and British and both result in more casualties and possibly prisoners leaving them less prepared for a counter offensive or forcing them to pull units and supplies from somewhere else on the front to strengthen the area once held by the now damaged or captured units.
Result:
France sues for peace, seeing the writing on the wall Italy joins as they are not going to face down Germany and AH that no longer needs to fight France and can turn their attention that way. UK indicates a willingness to negotiate, which in turn drags in the Russians (no way they are fighting Germany, AH, Bulgaria and Ottomans by themselves). This in turn drags in the rest of the CP (Bulgaria and Ottomans) and you have a peace conference. Would Switzerland be willing to host it as it is neutral in the conflict and next to both France and Germany along with Italy and AH. Negotiated Peace for the war.
Colonies- status quo ante, except for Asia were I see many of them being retained by Japan with payments sent to Germany. Main colony change I could see is part of the Belgian Congo going to Germany in exchange for a payment, which also acts as a means to held fund the repair of Belgium.
France- Minor border adjustments- any potential major gains in France (and Belgium) are given up in favor of the return of the German colonies, UK wants no part of Germany with a presence on the channel and keeping France intact, some reparations are also to be paid to Germany. I do expect the small coal area on the border to be put into German hands at the very least to deny it to the French and since it would be a minor adjustment. Restriction on Air Force possible
Belgium- Restored, possible sales of some of the Belgian Congo to Germany.
Luxemborg- Part of the German Empire (marriage to Prince of Bavaria was mentioned as occurring OTL earlier in the thread)
Italy- Reparations to AH (maybe Germany), relinquish claims in the Balkans
Russia- At this point they had been thrown back by the 1915 Eastern offensive by the Germans, so Poland is being formed in some shape, maybe some of the Baltic states. Pre war border between them and AH, likely the same between them and Ottomans (winter offensive has not happened yet), some reparations to Germany, AH and Ottomans
UK- Minor reparations perhaps, but mainly a status quo ante for them, they give up gains in exchange for the Germans not controlling the southern side of the English Channel.
Serbia- split between AH and Bulgaria (though some of it may be as a puppet state) but I know Bulgaria has some land claims on Serbia.
After the war-
The French are not ground down as much with only 1 and half years of war as they are OTL, on the flip side they have lost to Germany again, and this time they had an ally fighting with them and Germany had split her attention. They also now have to face a post war situation where they must repay loans from the US and give some funds to Germany as part of the peace, they still have a place in the World and still control almost everything from pre war, but gaining their claims on Germany is lost to them, perhaps for good. Their industry has been hurt badly with some of it being in areas that were either the front or overrun 1914. It will take time to pick up the pieces
The Italians- not good for those in power, they shed the blood of their youth and in exchange got nothing except having to give money to AH and give up their dreams in the Balkans, for now at least. I expect discontent with the government given the result.
Serbia- Now under AH and Bulgarian control, their dreams have ended, for now. I expect unrest to continue but surrounded by unfriendly powers and with no aid coming their economy is likely to shrink and maybe collapse, don't expect AH to do any development in their area of influence, instead more likely to try and extract funds.
Russia- They gambled and lost, Poland and perhaps more have been lost to them, their armies bleed and money being sent to their rivals. Unrest will be brewing, the prestige of the army has been destroyed (No Winter offensive against the Turks, no Brussilov Offensive) and two lost wars. Still Ukraine remains a part of the realm, and likely Baku (though there may be a push by the Germans for some of the Oil to be sent to them as part of the peace).
UK- Their youth have been bloodied (Autumn offensive, failure at Gallipoli) but their Empire is undamaged, Egypt and the Seuz remain as does the Jewel that is India. On the flip side loans are owed to the US and payment will be expected, the failure of their own land operations and those of their allies casts a pall, and their is still the matter of the restive Irish.
Germany- Things are good, superiority has been proven and some gains were made, Russia and France have been humbled on the field of battle with a slight weakening of Russia with the loss of Poland. Their industry should be intact while the French industry has been damaged. Alliances have been made and strengthened and opposition has been weakened. A connection from Berlin to Baghdad has been realized and with it the possibility of Ottoman Oil to fuel Germany industry and military.
AH- A mixed bag, Serbia has been dealt with, but the poor showing against the Russians in 1914 needs to be dealt with, Italy is still a threat and now on the opposite side. Bulgaria is a power in the Balkans for good or bad (good if they can work together). Still Russia has been bleed, and worse then AH, plus they have lost Poland and revolution cannot be ruled out. They have a solid friend and ally in Germany so their potential for a better future is there.
Ottomans- No gains in terms of land, but no losses either, they have had success and failure against Russia and British forces so more work on the military needs to occur. And the Capitulations are now a thing of the past. A potential friend has been made in Germany and AH, and there is oil to exploit, the future is what they make of it.
Bulgaria- A brighter future is here, gains have been made in Serbia, Russia is humbled and the Ottomans have turned away from the Balkans. The Italians have been shut out as well for now and they lay astride a line from Berlin to Baghdad and the oil and other trade that may entail. Opportunity is theirs.
Scenario 2: Neutral Italy, CP Bulgaria
Why- Better performance by AH against Serbia and Russia convince Italy that AH is not easy pickings and convinces Bulgaria that the CP are a good bet so they jump on Serbia earlier. Knock on effects are more forces arrayed against Russia, and Russia worse off, France can't remove all of their troops from the Italian border so a bit less forces in the west. Perhaps a spring Russian offensive with a better start position for the Austrians ends in enough time to move German forces west to meet the Entente autumn offensive and turn it into a counter offensive.
Peace- similar to the above, though no Italy involved in the negotiations.
Scenario 3: CP Italy and CP Bulgaria-
A better CP 1914 convinces the Italians that France is on the way to a loss so they join in 1915 in the efforts to make some gains. With additional French forces needed along the Italian border and Bulgaria also likely jumping in earlier to be on the wining side Germans manage the breakthrough they need to reach Paris. A side effect is this is likely to paralyze the Entente armies in the colonial sphere to adjust for the Italian Colonies and Colonial Armies, if this is after Gallipoli has started it makes it worse for the Entente as their free forces are tied up with the Ottomans
Peace- Some French territory ceded to the Italians, some reparations.
Actually in looking it over, if we keep true to the OP of Germany eking out a decisive win the peace is likely to look similar. With difference being who has to give money and who gets money, Italian CP likely gets some border adjustments with France and some of the Balkans (Albania/Montenegro). Italian Entente has to pay reparations and deal with the fact that they sent a lot of young men to their death only to have to make payments to AH on top of that.
Any bigger changes are precluded to me due to the eking a decisive victory, as opposed to a decisive CP victory which can look quite different and produce a different peace.