Is it permissible to accept attorney fees and recover money paid in an unlawful ("devious") manner in an installment sale case, and what is the Shar’i evidence for it not being due, if any?

It is permissible to sell a commodity for a deferred price that becomes a debt owed by the buyer, and the buyer must repay it as long as he is solvent. Procrastination in repayment is forbidden. The creditor may demand the debtor to pay the installments of the debt and charge him for what he lost in the process of collecting the debt, such as transportation fares, the cost of papers, and litigation expenses, provided that the expense is incurred in the usual manner. So, if you appoint a lawyer, you may take his fee; otherwise, no. If you cannot recover the transportation fares and the cost of papers except by stratagem, there is no harm, provided that you only take what is rightfully yours. However, what you expended in that regard does not entitle you to file a new lawsuit with the other document.