All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
If the husband gave his wife's brother this amount of money as a loan, then, in principle, the borrower is the one who has to repay it, and his sister is not required to repay it unless she guaranteed paying the money and her brother failed to settle it at the due time. If the woman settled the debt because she guaranteed paying it or if she did not guarantee paying it but she had done that willingly, the borrower is acquitted of the debt he owed to his sister's husband. However, he would be indebted to his sister if she paid the money with the intention that he would repay her.
On the other hand, if the husband forced his wife to settle the debt of her brother, he has to return it to her, knowing that being the sister of the debtor does not entitle the creditor to force her to repay her brother's debt. So, the husband fear Allaah and return the money to her, then he should try to take his money from the real debtor.