We learnt about this in Vet. Professional Studies. All vet surgeries, if you explain your problem will do the surgery and allow you to pay it off gradually. Remember vets (on the most part) become vets because they love animals, and alot of the time are willing to cut costs of surgery to clients. They should definitely let you pay it off gradually. If not, find a new vet!
EDIT: stupid me didn't notice the second page of posts. GO NAN! Also, find a new vet quick smart 'cause they shouldn't demand you pay up front.