Five years ago I worked in the United States of America (USA) and one day my ex-employer decided to lay me off (fire me). I thought it's completely unfair to fire me with no notice and without clear reason. I felt so hopeless and frustrated. So I managed to transfer $5,000 from the company’s account to my account under the feeling that this is my right (as this figure is 2.5 months of my salary) as compensation. Anyway, I transferred the money and cash it in 2 days and then I went home but the company (my ex-employer) found out about that transfer and held it through the bank after I already took it after holding $500 of my money. So now I owe the bank not the company and I owe them an amount of $4,500. Some months later I felt guilty but I only got $1,000 from this money and I sent (wired) the money to my bank account so now I owe the bank $3,500.
Now I'm back to right thinking and I'm quite sure that this money is "Haram" and I wanted to send them back the money. So I called the bank in order to refinance my debts (i.e. pay less money as a discount, agree to pay it over a period of time on monthly installments....etc.) and the bank told me that they couldn't find any details on my account as it's closed and does not exist any more and most probably, the money was written off. I swear I tried to help them find details on this debt but they couldn't. I know that I have to return the money and the question is: Could I give away the equivalent of the $3,500 in local currency to poor people, give it to an orphans’ institute, a mosque, etc.? (Sadaqa) or shall I return it back to the bank? I really appreciate your prompt reply as I'm so scared to die in debt.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is unlawful for your employer to take your money without any legal right as it is unlawful for you to take the money of the bank without a legal right.

Now you should do two things:

First, repent sincerely from what you committed.

Second, try your best to return the money to its owner.  If you could not do so, then you should spend this amount in good acts like giving it to the needy or the poor or orphan’s institutes etc.

Your aim in all this should be to get rid of this illicit money and the reward is to its owner.

Allah knows best.