What is the ruling on collecting a sum of money to pay off the debt of an absconding debtor who has not settled his debt, and is it obligatory to return the money to its owners, and is it permissible to use this money to settle other debts?

As long as you have given the money to the person for whom you collected it, so that he may pay off his debt with it, you have done what is required of you. You are not obliged to return the money to its owners or to pay off the debt from your own money, because a trustee is not liable unless he transgresses or is negligent. The responsibility lies with the debtor who took the money and did not spend it in the manner specified by the donors.