Had made arrangements with them but they dishonored them and kept on deducting amounts we never agreed on and on awkward dates

Had made arrangements with them but they dishonored them and kept on deducting amounts we never agreed on and on awkward dates. I raised the issue with them several times but they still could not resolve it, instead they keep on calling and being rude. Now they sending a letter threatening to hand over the account and list on the credit bureau. This is absurd ! I've been trying to get them to get the correct debit date and amount for months now!