How can something taken unjustly from work more than 15 years ago be returned, given the death of the Christian employer and the inability to locate his family after moving to another country?

Whoever unlawfully takes money must return it to its owner or their heirs. If they are unable to reach them after searching, they should donate it on behalf of the owner. If they find the owner later, they should give them the choice between approving the charity or returning the money. Repentance, remorse, and a firm resolve not to return to the sin are obligatory. There is no difference between the money of a Muslim and that of a non-belligerent disbeliever. Therefore, you must donate the money on behalf of its owner, so that whoever among their heirs embraces Islam may benefit from it.