I am not sure if this is an intentional game ability or simply a mistake. If you have very high deplomacy, you can trade money for money and send less than you get. Several things are affected by this though. First they must like you. (warm or close works best) You must also be alligned fairly close to the race in question. (IE... good to good, evil to evil etc). Also, if the race is intelligent or above I have not seen but a few successful attempts.
If this is intentional game play then so be it. I mean if your society is so smart that they are able to talk another race into the benefits of less of your money for more of thiers, then I guess you deserve the rewards lol.
But I think I am more inclined to think that this is mistake in the game, thus this posting.