The bank that owns the ATM has all the record for that day. you can visit that bank too. how did they equalize their accounts for showing 500 but paid 100. there will be a difference of 400 in one of the bank. and when you withdraw the amount, did you receive the receipt for that or not. keep it with you and complaint again.

Don't leave them like this if even the amount is small, coz this is cheating.

best of luck