What is the ruling of the Shari'ah regarding the repentance of someone who embezzled funds and is unaware of the amount, is unable to seek forgiveness from the owner, and does not possess sufficient money to return them? Is his repentance accepted? And what can he do in this situation?

One must repent to Allah Almighty and return all the money taken. Repentance is not complete without doing so. If the amount is unknown, it should be estimated cautiously and based on the preponderant opinion, and the larger amount should be paid, due to the Hadith (definition): "Whoever has wronged his brother concerning his honor or anything else, let him seek his forgiveness today before there will be no Dinar or Dirham." Whatever cannot be returned now becomes a debt that must be recorded in a will, due to the Hadith: "It is not right for a Muslim man who has something to bequeath to spend three nights without having his will written down with him."