If you are no longer fulfilling your part of the bargain the other party should expect you (and you should have the option) to reimburse them. If they paid 250 gold for a luxury resource for 30 turns and only got it for 6 the standard result should be for them to demand (at least) 200 gold back from you or another equally useful resource (possibly subject to negotiations).
The diplomacy is really atrocious, and though the trading system is at least slightly better than the stupidly annoying begging that starts as soon as you declare friendship with someone where an AI player who is clearly ahead of you in every area comes asking for favors they don't even need because now that we're friends you should want to help the one who is less fortunate. If they want to help you in return? Clearly you don't understand how this works - you pay us, we don't pay you.
I've stopped accepting offers of friendship only because of this, and the fact that sooner or later one of your friends will denounce you which will reduce everyone else's relationship with you even though they did it for completely selfish reasons.
*gasp pant sigh*
Okay, rant over. I can has patch now plz?
