Does the sin of a usurious loan fall upon the father who took it to marry off his son who is compelled to marry, or upon the son?

Borrowing with usury is religiously forbidden, but the borrowed money becomes the property of the borrower. Both the borrower and the lender must offer sincere repentance. The lender must not reclaim more than his principal, as Allah Almighty says: "But if you repent, you may have your principal – [thus] you do no wrong, nor are you wronged." There is no harm in using this money to marry off one's son, and there is no harm for the son in accepting it if he is in need.