In the UK, as long as you have a bank account linked to your PayPal, they will attempt to automatically transfer money from your bank account to cover the negative balance. This only works if you have a direct debit set up - they can't automatically take money from a savings account, as it does not have direct debit facility. They will only attempt to collect the full amount, so if you owe $900, they will try to transfer $900 in one go. If you have $899 in your bank account, the transfer would fail and the money would remain where it was.
Just pay off the debt in installments - no need to do it all at once. Although, they can report this to credit reference agencies, if this is your real account.