Does the seller have the right to annul the contract of sale of a building on which there is a mortgage loan that the absent buyer has not repaid for 11 years, and is he obliged to complete the sale according to the contract after the buyer reappears claiming it?

If the sale contract has been concluded and a portion of the price remains as a debt owed by the buyer, the sale is valid. The transfer of real estate ownership in the commercial registry is not a condition for its validity. Therefore, the seller does not have the right to withdraw or annul the contract. Contracts must be fulfilled, as Allah Almighty says: "O you who have believed, fulfill [all] contracts." Justice must be upheld even if the buyer fails to fulfill some conditions, as Allah Almighty says: "And do not let the hatred of a people prevent you from being just. Be just; that is nearer to righteousness."