"If you trade a resource or GPT to the AI and receive a lump sum from the AI in return, you may not DOW that AI player until all of the turns of that trade have completed."
That sounds reasonably fair, but i think there should be some sort of clause or exception, again with it's own specified rule, regarding situational politics. It's good practice to avoid situations where it becomes necessary to DoW to save yourself from something, but you can't always predict the future, especially in marathon games (my preferred type). I try to calculate a reasonable amount of the lump sums difference and, if I have the funds, return the due amount to the civ right before DoW. a simple calculator will do the trick, something I know every computer has.
round to the nearest whole number, round up at 0.5
Lump Sum = X, length of deal = Y, number of completed turns + 1 = Z, I say +1 because the deal does not complete on t90 (marathon), it is "finished" on t91
((Z+1)/Y)*X = what you pay the Civ before DoW.
Example:
lump sum 250g, completed turns 47, length of deal 90
(47+1)/90 = 0.5333, 0.5333*250= 133.33, round to the nearest, and you owe the Civ 133g before DoW. Not hard once you get into it.
Does that sound like a reasonable clause in case you have to (or want to?) DoW prior to the deal being completed?