To add to what has been said:
you might want to consider going to the preferences menu and enabling the "always renegotiate deals" or whatever specifically its called.
This means that after 20 turns, every deal (other than peace or war) will come up for renegotiation.
2 benefits:
1. The AI does this automatically-if the trade is to his advantage, he will keep it, if its to yours, he will end or seek to alter it.
2. you don't have to keep as close a watch on your deals and their expiration dates.
Frequently I am able to renegotiate a deal for more money
Enjoy the game