The idea of buying and selling territory is way, way overblown in my opinion. As far as I can see, states sold overseas territory for two reasons:
1. Their claim over the territory was almost purely on paper only, and they were in reality selling vast amounts of wilderness and lands inhabited by tribes not acknowledging their authority.
2. They were about to lose the territory anyway, and figured they might as well get some money for it, or they chose to trade it away for other territory in negotiations ending a war they lost. See the Mexican-American War, and various colonial wars.
Alaska was a combination of 1 and 2, the Russians thought the British would seize the territory sometime soon.
There is also swaps like Zanzibar for Helgioland. Note: I have canuck friends who claim UK sold them out on Alaska border for concessions elsewhere.
Now to the main idea, I can't see the USA buying all of Canada expect as some fig leaf after some a major win in some USA/UK war. So just to pull out something, the USA joins CP in WW1 for whatever reason, and takes Canada. Then, sure the USA might "buy" Canada in a peace deal, but the types of things I can see happening.
1) Cash is exchange in relationship to the Maine border dispute. Maybe same as OTL, maybe bit bigger USA.
2) Could see cash exchange in Oregon settlement.
3) Like the purchase of Southern Arizona, the could come something the USA needs a small border adjustment on for some reason. Cash is exchanged for mostly empty land.
4) USA gets empty land like central Canada plains. If you get a POD where these area is less developed (no trans Canadian RR), then it becomes lot more likely.
5) USA had concession on things like landing to dry cod in Canada. I could see this being trade if the USA wanted something in particular and it was small enough. Say, if for some reason USA coveted part of Yukon territory that does not contain Gold or wants northern half of Vancouver Island or something else small.
6) USA had concession on part of what is now Malaysia. Maybe trade for something closer to USA. Since this is near the time of the Alaska deal, I am not sure say part of Borneo for Yukon territory is ASB on the UK side. Or part of BC for fishing rights. Or slightly expanded Alaska. Just avoid major Canadian population centers.
But for the big concessions, you either need UK fear war with USA which is probably dual POD involving USA avoiding Civil war and UK need help somewhere else, which sounds a lot like USA avoids civil war and WW1 comes early.