He took a bank loan and his father benefited from it; may the father add money at repayment?

Shalom,

He took a bank loan of 40,000 shekels for renovations. His father, who needed the money for a short time, asked him for the money so that he could pay the contractor at the appropriate time. Now the son is lacking 3,000 shekels, and his father wants to make up the difference for him – both as an expression of gratitude for the loan and because he wants his son’s benefit, and he would have given him this even without the loan. Is this permitted?

Shalom u’vracha,

It is permitted for the father to give his son this additional amount, provided that he does not give it to him together with the repayment of the debt.

In Siman 160, se’if 7 it is explained that if the borrower customarily does a favor for the lender, he is permitted to do so even during the time of the loan. However, there are two conditions: (a) it should not be done in public, and (b) the lender should not take the favor on his own initiative. Since it is customary that a father helps his son when he needs it, there is certainly such a regular pattern here, and in addition this is not done publicly. Therefore, he is permitted to give the money; however, at the time of repayment of the debt there is a prohibition because it appears like interest (machzi k’ribit), and it is forbidden to give additions, even if he gives them for other reasons.