Does an employee's debt for an advance payment that was not deducted from his salary before his employment ended become absolved, and what should he do with it if it is impossible to return it to the company?

You must strive to reach the owner of the company to repay the money. If you are unable to reach him, then give the money in charity on his behalf, with the guarantee that you will compensate him if you find him in the future. Ibn Taymiyyah said: "The unknown is like the non-existent in Sharia, and that which cannot be reached is like the non-existent."