He pilfered some money and does not know how much, and he cannot repay it. What should he do?

During some part of my life I was a sinner, and my problem is that I used to pilfer from the petty cash in the place where I was. After Allaah guided me I became aware of my sin and I started to give back the money I owed. But the problem is that I do not know exactly how much it was. I tried to get the owner of the place to forgive me without mentioning the reason but I could not do it. I do not have any other money that I could use to pay it off. Will Allaah accept my repentance if I cannot return what I owe? If there is anything that I can do, what is it?.

Praise be to Allaah.
What you have to do is to repent to Allaah and return all the money that you took. Your repentance is not complete otherwise. If you do not know the exact amount, then it is sufficient to do what you think is most likely, and be on the safe side [?] when estimating how much you owe. If you are not sure whether the amount is eighty or one hundred, for example, then pay one hundred, so that you will meet your Lord free from the burden of this treachery and wrongdoing. Al-Bukhaari (2449) narrated that Abu Hurayrah (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Whoever has wronged his brother with regard to his honour or anything else, let him settle the matter now, before there is no dinar and no dirham, and if he has any righteous deeds some will be taken from him according to the wrong he did, and if he has no hasanaat (good deeds), some of the other person’s sayi’aat  (bad deeds) will be taken and given to him.”

Whatever you are unable to repay now is a debt that you owe, and you will not have discharged your duty until you pay it. What you have to do in that case is to make sure that it is mentioned in your will, lest death come upon you suddenly before you can pay it off. Al-Bukhaari (2738) and Muslim (1627) narrated from Ibn ‘Umar that he heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “No Muslim who has anything concerning which a will should be made should spend three nights without making a written will.” ‘Abd-Allaah ibn ‘Umar said: I never spent one night since I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say that but I had my will with me.

We ask Allaah to help you and give you strength.

And Allaah knows best.