Must the thief sell the land he bought with stolen money to return the stolen money to its owners, or is it permissible for him to keep it until it profits the amount and then return it? And does the land become permissible for him after repaying the debt?

Repentance from theft is not complete except by returning the stolen item to its owner or obtaining their forgiveness. The land belongs to the questioner, and they are not obligated to sell it in principle, because the forbidden relates to the obligation (dhimma) and not to the specific asset itself. However, they must hasten to return the money to the aforementioned company. If they cannot do so except by selling the land, then they are obligated to do so to expedite the return of the money, and it is not permissible to delay while waiting for profits.