What I have in mind is that they would be able to use "occupation costs" for rather more than just direct occupying forces, provided they keep sufficient forces in Germany. These "occupation costs" would essentially be set at what the German defence budget would otherwise have been, and in return Germany is forbidden from having armed forces and the occupying powers will base forces in Germany to protect it and treat any attack on Germany as an attack on themselves. That gives them a strong incentive to keep the occupation going, since it would provide rather more money than it actually costs once Germany starts to work as a country again.
The cash transfer from Germany wouldn't be set to any particular cash value, but rather would be a small percentage of GDP - I'm guessing about 3% or so - and paid in Germany currency to ensure they can't cheat with inflation, etc.
I'm thinking along the lines of 3% since it's the approximate peacetime military spending of the UK and France so is hardly an unreasonable cost of defence, and the Germans have been spending many times that for a decade now so can't claim it's unaffordable. If they can make Germany work as a country again, it's also worth shedloads of cash - in current day money it would be over $100 Billion. They're also assuming a long-term commitment, so ITTL would still be paying it right now.