In the Name of God, the Most High
1- What is currently being carried out in the Iranian banking system is not based on fixed interest considering the agreement signed between the bank and a client. The legal frames of bank transactions in Iran are Shari’ah compliant and there is no objection to them as per the Islamic law. If at the time of calculation of interest a fixed interest is supposed, it is a practical offense and there is no problem in it, if it is not conducted in accord with the agreed-upon terms and conditions. However, this has no role in the validity of the bank transactions insofar as they are conducted according to interest free transactions.
2- If you sign contracts similar to those existing in the rules and regulations regarding interest free bank transactions, there is no problem but if you act against the law and do what has been prohibited by bank rules, it is not allowed. Apparently, the supposition made in the given question is among the non-Shari’ah compliant transactions.