I'm so sorry to hear the news, first of all, I would inform the breeder of the news, a responsible breeder might even help you with the cost of surgery or return your buying fee. Not sure what state you live in, but many states have puppy lemon laws.
Not all liver shunts are repairable by surgery, those are the types where diet works okay, but if you dog has the type of shunt that is repairable, I'm not sure diet will help much, it's best to get the surgery. Maybe others can help you more with that. When you say recovery rate is 75-85%, that sounds like great odds to me, I understand the financial limitations, but have you considered credit care?
Veterinary Financing | Healthcare Financing | CareCredit. I'm hoping you can get the help that this pup needs, if not, maybe you should considered giving him to a rescue. I know this is painful to even think about, but sometimes you have to do the best thing for them, even if it's painful.