Should he lend money to his sister when he knows that she is going to pay back the loan with haraam money?

Interest Bearing Loan to Get a Better Job

Is it permissible for the borrower to promise the lender a gift when paying off the loan?

It is not permissible to dispose of wealth given as a waqf for the mosque by lending or borrowing

Paying interest because of inflation is riba

Buying a foreclosed house if it is sold at auction because the borrower is not able to pay off the riba-based loan

Ruling on borrowing from one whose wealth is haram

Ruling on giving a gift or doing a favour to the lender when paying off a loan

Is it permissible for his wife to take a loan from the Bab Rizq program in order to give him the money?

He is embarrassed to return the money to his neighbour because it is such a small amount