In a contract each side must give some consideration to the other. Often
referred to as the quid pro quo - see the Latin terms below. Usually this is the price paid by one side and the goods supplied by the other. But it can be anything of value to the other party, and can be negative - eg someone promising not to exercise a right of access over
somebody else's land in return for a payment would be a valid contract, even if there was no intention of ever using the right anyway.