Any kind of commercial exchange or bargain. By the 14c it signified *usury, as increasingly merchants were the great lenders of money after the expulsion of the *Jews in 1290. Protection by the pope (as formerly usury was condemned by the Christian Church) assured its 'legality' - the Church itself and its magnates were themselves all heavily in debt. The Latin form was chevantia.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.