I would say she does look slightly...off but again, she might just be a rare color. She's so young it's hard to tell. How old is she? And again, why do you think the breeder lied?
The mom being a white Boxer might account for this odd coloring. I'm going to say right off that this wasn't a good breeder if the pup's mom was white. Good breeders do NOT breed white Boxers. A lot of them even cull them out at soon as they're born. Aside from the fact they're a breed fault, they are genetically weak like @annageckos said, because they are VERY prone to deafness and blindness. Those dogs do not carry any genes you want to pass on.
Did you meet both the parents or just know their coloring? Unless one of the parents looked especially not boxer I'd say she's just a boxer not bred to breed standard very well. I feel like boxer/pit mixes almost always look more pit than boxer to me and have longer muzzles than she does. Fawn boxers do sometimes have less of a mask, also, although it isn't to breed standard.
My wife got her and saw the mom boxer which was white. We love her to death just wanted to get some opinions. We might do DNA to get a better idea. I know it doesnt prove purebred but it could clear up any questions.