[On a Guardian’s Demanding Money for Himself Apart from the Dowry2]

Question: Two men, each of whom has a son and a daughter, agreed that each would marry his daughter to the other’s son, with the dowry for each girl specified—i.e., this is not shighār. They wrote an agreement between them that if the son of either of them and the daughter of the other could not get along and he divorced her, then the other man—whose son did not divorce—would bear a sum of money to help the one whose son divorced to marry another woman.
In fact, one of them divorced the other’s daughter, so he required him to bear about forty thousand riyals, and several years have passed. Must he return the forty thousand to its owner? Or is their stipulation valid since they both consented?