Is a commission of $10, which sometimes increases to $50, for transferring an amount of $100 through the bank, permissible or forbidden, knowing that the amount is not received from the merchant until after the transfer is made, and the merchant's payment of it may be delayed?

Lending to a merchant for a commission that increases or decreases depending on the delay in repayment is usury (riba). The proof of this is the consensus of scholars that stipulating an increase in a loan is usury. As for the commission for a bank transfer (hawala) that a bank takes, there is no harm in taking it from the merchant. It is like the fee of a measurer or a weigher and is incumbent upon the debtor.