Is it permissible for the seller to back out of a house sale after receiving half the amount and handing over the keys, and what is the ruling on compensation if they refuse to complete the sale?

If the sale fulfills its conditions and pillars and the seller does not have the option to revoke it, then the sale is valid and enforceable, even if the entire price or part of it was not delivered at the time of the contract. However, if the seller stipulated the option of cash payment and the buyer did not deliver it within the specified period, then the seller has the right to annul the contract. The seller does not have the right to withdraw from the sale unless he has the option of cash payment or another option. If the buyer wishes to cancel the sale for a consideration, there is no objection. It should be noted that usurious loans are forbidden.