There should be a facility for delayed and rateal payment in the Diplomacy window.
When you select "Give gold" there should be 2 more field: Turn of payment (current turn by default) and number of turns (payment will be done each turn for x turns). A check box could be implemented to select if the amount is to be multiplied for Turn# or is already the total amount. Then, the server should take care of administrating delayed payment on each TC (beginning of turn). Actions should be taken if payment becomes impossible.