Unacceptable

I have a standard bank car loan,apparently they use the 25th as the debit date order,which i wouldn't agree on since i only get paid on the 27th. 4 months later they contacted me saying i was not able to debit my account since there is no money,obviously... so they told me there would could be a increase or decrease in the monthly debit order if i changed the debit order date. Not acceptable at all. Really, 2 days,which i didn't even agree on the 25th anyways????