peewee - Do not under any circumstance pass this debt agency any details of your debit card, once given they can subtract money at their leisure and amounts to suit themselves, they would probably clear out the money you need to live on (they will take all the arrears for starters), there is nothing you can do about it once card details have been given voluntarily, except cancel the card altogether with your bank....too late once they've got your money. !
Instead if you must pay, write them a cheque or even get a friend to do this and pay the friend in cash instead...or set up a standing order S/O on your account, that will still let you maintain full control over the amount and the frequency of payments.
Could I ask who the bullying debt agency is and what the debt is for that they are chasing you over and any other details you would care to supply.?
There may be other alternative methods to deal with this, that could possibly be to your advantage. !