Adjust trader gold offer during trading

I like to give the traders profit when trading, but when they have a huge pile of coins, it seems impossible (?) to do this, since they will always offer coins to make up the full price of whatever you're giving them. The only workaround seems to be to sell items for all their gold and then sell them more stuff with their goods making up less than the full price.

It would therefore be good if we could reduce their gold input (and ours?) to enable more productive bartering.