He agreed with the bank to give him a sum and collect it from his debtor after adding a certain interest

A: It is impermissible to do so as it entails an obvious Riba (usury). This is because the bank in this case will get back the entire amount it guaranteed from the person indebted to you, which is more than what it gives you. In fact, doing so involves both Riba Al-Fadl (usury of excess, selling an item for another of the same type, on the spot, but in excess) and Riba Al-Nasi'ah (usury of delay, conditional excess for delay of payment). Therefore, it is impermissible because Allah prohibited Riba and seriously threatened those who indulge in it of a grave punishment. Moreover, the Prophet (peace be upon him) cursed the accepter of Riba, its payer, the one who records it and the two who witness to it.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.