BMW finance not taking responsibility for bad service

I traded my BMW at nissan. the BMW was settled in July by nissan all the time I fought with nissan BMW was at fault I didn't know . BMW finance deducted my monthly installments wrongly and haven't paid me till today. No communication from them. It took them a second to deduct my money but its taking month's to figure out when to pay me back regards innocent modise ********** 098