Lender referring a third party to debtor for extra money
Question

                Iloaned someone ten thousand riyaals, but he is delaying repayment and has
      not yet paid back what he owes. I owe a third person ten thousand riyaals,
      and he came and asked for his money. I told him, ‘Go and ask So and so
      – i.e., the first person – for the ten thousand riyaals that I owe
      you, for the ten thousand that he owes me, and I will give you two
      thousand riyaals extra, to make up for the length of the delay. In other
      words, twelve thousand riyaals in return for ten thousand. What is the
      ruling on that? Please advise me, may Allaah reward you.
Praise be to Allah.It
      is not permissible for you to refer the one who loaned you ten thousand
      riyaals to the person who owes you money and is delaying repayment and to
      add two thousand riyaals to make up for the lengthy delay, because this
      extra amount is ribaa, and ribaa is haraam according to the Islamic
      sharee’ah.
And
      Allaah is the source of strength. May Allaah bless our Prophet Muhammad
      and his family and companions, and grant them peace.