Is it permissible for someone who stole money and repented to place the stolen amount in a mosque with the intention that it belongs to the owner of the money, due to the impossibility of delivering it directly to him?

The money must be returned to its owner if he is present, known, and accessible. It is not necessary to go to him in person or inform him of the truth of the matter. Rather, the important thing is to return the money to him by any possible means.