If funds are an issues, you can do what I used to do before BRAT came along.
Find out the vet of the family your placing him with, have the family pay you for your expenses and then take the dog to the new familys vet for the surgery to be fixed. Drop the dog off the day of the surgery and then new owners pu the dog.
They get to pay the bill.
That makes sure the dog is fixed, sets up a new vet record for the new dog in the family, and you don't have to pay anything out of pocket.
Mostly, you know the surgery WILL BE DONE.
Thanks for making sure he is fixed before rehoming.
That is so important.