What is the ruling on someone who took a loan and vowed to return the amount with an increase as a "vow" (nadhr)?

This vow is not valid because the one who made the vow vowed two things: a vow of an obligation (repaying the debt), which is obligatory whether he vowed it or not, and a vow of disobedience (an increase on the debt when repaying the loan), which is a loan that draws benefit, and this is an act of disobedience. Therefore, the vow is not binding according to the majority of scholars, and nothing is incumbent upon the one who made the vow. Furthermore, it is unanimously agreed that a vow of disobedience is not permissible to fulfill, based on the Prophet's (peace be upon him) saying: "Whoever vows to disobey Allah, let him not disobey Him," and also: "There is no vow in disobedience to Allah, nor in what a servant does not possess." Al-Albani authenticated this narration. As for a vow of an obligation, it is not binding because it is invalid to commit to something that is already incumbent.