Is it permissible not to fulfill a vow to build a mosque on a plot of land if the family demands selling the land to pay off debts and build housing for the children, knowing that the reason for the vow was a large debt incurred from guaranteeing someone?

A vow (Nathr) is initially disliked, but it becomes obligatory to fulfill it if the condition upon which it is suspended is met. It is not permissible to revoke it if one is able to fulfill it. So, if Allah saved you from your problem, then fulfill your vow and build the mosque with the value of the land.