I attend the mikvah sometimes before shachris etc, however the local mikvah’s payment machine seems to have constant problems resulting in me often being unable to pay (yet the gate is unlocked, i.e. they allow you to enter). I have really lost count how much money I owe them. What is required of me al pi halacha to pay them back?
You should try to estimate how much money you have to pay and add money to that to make sure it is enough and pay it to the mikvah
Igros Moshe CH:M 1-88 (12)