What is the ruling on paying the value of a post-dated or immediate check for a commission, and what is the percentage that Islam permits for this, and is there a time limit for the permissibility of cashing a check for a commission?

Selling a check for less than its value is usury (riba). However, it is permissible for the owner of the check to authorize someone else to cash it from the bank, and to pay them a fee for that without a fixed limit. Rather, the matter depends on the mutual agreement of both parties, on condition that the agent is not given the amount of the check until after it has been withdrawn.