What is the ruling on collecting debts between the seller and the buyer for a certain percentage, and is it permissible to impose another percentage on the debtor for expenses?

It is permissible to work in debt collection for a known sum or a percentage of the collected amount, provided that the debts are not usurious, and that both parties agree upon the fee amount and the time of its payment. The procrastinating debtor bears the costs incurred by the creditor to collect the debt, but the insolvent debtor is not obligated to pay anything. It is not permissible for the collector to take fees from the debtor, unless the creditor imposes the collection fee on the debtor. Lawyers are not permitted to exploit checks or debt instruments to unlawfully take money from the debtor.