What is the ruling on a penalty clause when selling a commodity in installments at a specific price, with an additional amount stipulated as a penalty to the lawyer in case of non-payment, and is it permissible to take this amount if the case is followed up personally?

It is forbidden to impose a penalty clause for late payment by the debtor, because it falls under the category of explicit usury (riba sareeh). However, it is permissible to obligate a defaulting debtor to cover litigation costs if suing him becomes necessary, provided that no more than the actual cost incurred in recovering the right is taken from him, and this should be done in the customary manner.