How can one get rid of the sin of an interest-based loan, and is it sufficient to repay the principal amount to the bank without the interest to avoid the sin, and when exactly does usury occur: at the time of signing the contract or at the time of paying the interest?

Usury is a grave sin. Whoever is involved in it must repent sincerely and attempt to annul the transaction by returning only the principal, without the interest, as Allah Almighty says: "...But if you repent, you may have your principal. You do not wrong, and you are not wronged." If you are unable to do so, and refusing to pay the interest would cause harm, then it is permissible to pay it to avert harm, along with sincere repentance. This would lift the sin, and "Allah will change their evil deeds into good ones." Usury is an increase taken without compensation. The sin falls upon the one who consumes usury, the one who gives it, the one who writes it, and its two witnesses; all of them are "equal."