I am confused on why you need to breed her at all.....since you only want a pup to replace your older female when she passes, won't this pup do for those purposes alone? Why do you need 2? It seems ridiculous to me to buy a pup, so you can breed it after the old one dies, so you can just have another pup....
Also my 2 cents are, you could ask for a small portion of your fees to be returned to cover her now necessary cost to be spayed, and to cover the inconvinience of not being able to get a litter, but that is all. I certainly wouldn't expect a refund, esp since she is so far a healthy pup.
I am also a bit surprised by the breeder, here most Boxer pups come with a no breeding clause in the contract.