Is what happened to the questioner—in terms of problems, distress, worries, and the loss of the project—caused by dealing with an interest-based bank? And does the sin fall upon him, or upon the bank, or upon both of them? And is one who deals with interest-based banks only to receive his salary considered sinful? And must he move to an Islamic bank after repaying the loan, or is it permissible for him to remain with the same bank without taking another loan?

Your feeling of guilt and shortcoming is appropriate, because usury (riba) is one of the greatest prohibitions. Allah has warned those who deal in it of war from Him and His Messenger, and Allah destroys usury. The Prophet (peace and blessings be upon him) cursed the one who consumes usury, the one who feeds it, its scribe, and its two witnesses.

Usury is a widespread evil, and its consequences are severe. The distress you feel and the lack of blessing in your wealth may be due to your involvement with it. Therefore, hasten to sincere repentance. If accelerating the repayment of that loan would eliminate or reduce its usurious interest, and you are able to do so without undue hardship, then you should hasten to do so.

Furthermore, it is not permissible for you to keep an account in a usurious bank, even if you do not take out a loan from it, because merely depositing your salary there is an aid to the usurer in his falsehood. Allah has forbidden cooperation in sin and aggression, and there are legitimate alternatives such as Islamic banks.