I think its Yeidish for "fair gain".

The idea was that if two people tryed to decide on a fair price for some goods, and the discussions came to a dead-end, the seller would declare "Benemunes Parnusa", the buyer would look at the goods and say "you deserve to have X$ profit" (or something similar), and then the seller would say: "OK, the goods cost me Y$" The buyer would then pay X+Y$ and all would be well.

There was a strong social taboo against lying about the original price of the goods.

1 - http://techst02.technion.ac.il/~sheket/html/stance.h...
2 - http://haayal.co.il/story.php3?id=274#7228

cheech&chong - lets make a dope deal