I filed the banking data with my complaint in the first attempt and they paid the 250€ into my account a day after accepting the claim. Of course if the amount is not correct it will be a pain to debate since they already sent money.

Banking laws are different in various countries, the consumer is not always protected if someone does a debit transaction using your account number. (eg US ACH debit or EU SEPA direct debit). So a certain scepticism is understandable.