My question is; how does one purify himself or his place of business from things that may have occurred prior to knowledge that it was haram? For example, I own clothing stores, but have financed this business via moneys borrowed and paid interest upon. To this day I am using a bank where I have a loan and pay interest on it.
Also in the past I had a business that did not succeed, and was forced into bankruptcy, which meant that there were many people and businesses that were left unpaid, what is my Islamic duty in that case? I did make a sincere effort to pay all that I could but after the business went bad was not able to repay them.
It is wajib that you repent for each loan involving interest you took. The Prophet (peace and blessings of Allah be upon him) cursed all involved in the interest, the recipient, the sponsor, and the one eating and the feeding it, as stated in the authentic hadith. The Prophet (peace and blessings of Allah be upon him) said, "The gold for the gold, the silver for the silver, wheat for wheat, barley for barley, date for date, salt for salt, similar for the like, and hand in hand. So whoever adds or seeks to add, then he has used riba. The recipient and the giver of the transaction are the same". (Reported by Muslim, #1584.)
On the authority of Jabir (may Allah be pleased with him) who said that the Messenger of Allah (peace and blessings of Allah be upon him) "cursed the one who consumes interest, the one giving it, the one bearing scroll, and the two witnesses to it". He said (peace and blessings of Allah be upon him), "they are all equal." (Muslim narrated it.)
If a Muslim wants to partake in a specific affair and he doesn't know it's ruling, then it is upon him to ask before he gets involved in it. Ignorance isn't an excuse for each and every situation. As far as the loans you borrowed, you are obligated to return the initial money borrowed only. If you are forced to pay riba, and that is the added amount with no way out, then we hope that you do your best in repenting to Allah to pardon you. We also hope that you remain consistent in your business and give sadaqa with what you are able to, in order to purify yourself and your wealth.
We ask Allah to enrich us with His halal and to avoid His haram.