Is the puppy with a rescue or breeder? If it's the latter then I'd be very wary if they're still selling the pup, even at a lower price to their other puppies. If a puppy is born with any kind of health issue I'd expect a decent breeder to fully intend to keep the pup (whilst potentially remaining open to the idea of letting the pup go to the perfect home should the opportunity arise), not just to sell the pup anyway.
Presuming the puppy is white, they've also knowingly or unknowingly (neither being great options) taken the risk of producing white puppies despite their much higher rate of deafness than coloured Boxers.
My concern is that if they either don't care or don't know about the risks of deafness in white puppies and that other areas of their care or knowledge is also lacking.
Are both parents health tested? For Boxers this would be a minimum of hip scoring with a result under 14 (but preferably as close to 0 as possible) and heart screening for Aortic Stenosis with a grading of 0 or 1. Ideally they should also have had a DNA test for Degenerative Myelopathy (wither either one or both parents either testing clear or being clear by parentage) and elbow scores of 0. There's lots more to being a decent breeder than just health tests but they're a good place to start to weed out the less than ideal ones.
As has been said, Boxers are a very active breed and although deafness doesn't completely rule out the dog being able to go off lead you'd need to be confident you could still provide them with adequate exercise if you ended up unable to let them off lead. There are lots of secure fields available for hire these days, which are a great option if you have a dog who isn't safe off lead. There's a database of them here which is worth looking at if you are considering this puppy.
If you've not had a Boxer as an adult then it is jumping in at the deep end a bit to go for a deaf pup. There are lots of sites and FB groups devoted to owning deaf dogs, these are well worth seeking out and reading through as they can help give you an idea of what living with a deaf dog is like.