Is it permissible to request a specific amount and document it in writing as a commission for assisting in the recovery of an inheritance for which the commission was not documented in writing?

What is forbidden is lending money on the condition that it be returned with an increase, which is usury (riba). As for agreeing on a permissible transaction in exchange for a known profit or a known wage for effort expended, there is no harm in it. This is done through a ju'alah (reward-based contract) agreement. It is permissible to give the owner of the money a known percentage or a wage to someone who helps him recover his money. Ju'alah is permissible for returning lost items, runaway slaves, and the like. Abu Hanifa, Malik, and Al-Shafi'i held it to be permissible, and they cited as proof the Almighty's saying: "And for he who brings it [the lost object] is a camel-load, and I am responsible for it," and because there is a need for it in unknown tasks where an ijarah (fixed-wage contract) cannot be concluded. The compensation should be known, and it is permissible even if unknown, as long as it does not prevent delivery. The questioner should document this permissible contract.