Also, paying money to your friends is probably bad, psychologically. There is a "social mode" with family and friends, and a "business mode" when dealing with money. They use different rules. For example the business mode is based on principle that everything can be replaced and traded; but the family and friends are supposed to be special. Trying to calculate whether the X hours I gave to my friend really have the same value as the X hours my friend gave to me seems like a certain way to ruin our friendship.

(I am not sure how much this is culture-depended.)