Oh I see what you mean now. Like spid said could you agree a repayment plan with them? Perhaps you could make a lump sum payment to pay some off and then the rest by instalments?

If another vet has said there was no need to wait 4 days, could you get them to confirm this in writing for you? I would consider making a complaint to the vets about delaying the operation for several days through no fault of yours has therefore incurred further costs you wouldnt have had to pay and ask as a guesture of goodwill if they can write these costs off.

Just an idea!