Is it obligatory to return the percentage of money to the bank that paid a sum to one of the employees, then took a percentage of money from the payer at the end of the month?

The bank is not entitled to anything more than what it paid on your behalf for the procedures only, because its demand for an increase is pure usury (riba). If you are able not to pay the increase, then you are obliged to do so. However, if you are compelled to pay it, then there is no sin upon you for paying it, and the sin falls upon the one who compelled you, unless you were colluding with them beforehand to pay the increase. It is necessary to be cautious when dealing with banks again.