A "purebred Bullmastiff" breeder sold us a mutt dog....?

A couple of weeks ago we bought a dog from a breeder; drove 16 hours to get him, and paid $1050 for the dog himself. Since the day we got him I had a gut feeling telling me the dog isn't a purebred his features look nothing like ones i've seen before. He has no wrinkles, and his snout is too long. Hes 9 weeks old and looks nothing like a Bullmastiff. His coat is somewhat long and black, breeder said it was extremely rare and was shocked that ALL the dogs look this way. I took him to the vet for the first time yesterday and the first thing the VET says is "Why do they have you down as a Bullmastiff your no Bullmastiff!" I just looked at her and was like "you gotta be f***ing kidding me, he better be." She said "Well, I just saw one today and he looks like a black lab mix." This isnt the first time we heard this and now coming from the vet, I'm pissed. The dog is AKC registered although im not an idiot I know that even if they are AKC this doesnt exactly guarantee that the dog is from the parents the breeder says they are. Anyways I got a blood DNA test done at the Vet and will know in 2-4 weeks what his result is, although I am 90% sure he is not purebred. I am willing to work with the breeder because I honestly don't think she knew although by now she must have some speculations because she still has 2 of the dogs from the litter. Anyways I'm looking for some legal advice, I really don't want it to escalate to the law but I will sue her if needed. Also I live in SC ans she in TN, so thats the doozy right there a big ole' pain in my a**......... Thanks for the advice in advance.