Need to see the pictures to try and help. Also it's hard to tell what breed until the dog matures and is pretty much an adult.
I've had severe dogs that I thought were mixed breeds or ab crosses until they hit around ten months or so then they suddenly either started musceing up or their ears stood after the vet assured there was no possibility and the pup was definitely a mix. Surprise on me.
Many peope pass from their pups as pb and try and squeak momey from peope, they use friends addresses or will meet you somewhere or even bring the pups and the mom and sometimes dad too outside a store etc and have a sign up "pups for sale"
My first black German shepherd pup was purchased as s lab pup, he was only 7 weeks old but mom had 22 pups, couldn't feed them all so I brought my $5.00 ab home and watched him grow. At 10 weeks we noticed some darker brown coming in on his inside legs and some highlights on his face, he kind of resembled a little bear cub. The pup grew very long and lanky, his tail didn't seem to stop growing, it was almost down to the floor by 20 weeks. His markings were definitely changing and we brought him to the vet and asked what mix was he. Vet assured us that the markings were called 'seal' just a lighter faded black, mostly from poorly bred dogs. At 7 months the pup surprised us al, his ears just suddenly popped up, big triangular erect ears but very much puppy round face. At 10 months long curved tail almost touching the ground big a key strides in his ewe, downward sloping back from the shoulder, definitely not lab but was unsure what breed. When he grew in his spring coat after winter he turned into a mostly Black and Tan German shepherd. Never knew the breed before him, he had some brown highlights on his face, his owner legs were classic pattern for the tan of a back tan sadde shepherd, his belly had tan mixed in and the underside of his tail was aso mixed.
His sister was purchased by a woman a bock down from us, she traveled with her dog so we seldom saw Kasey. I was shocked to see she had faded from s pure black butterball pup to a back and silver shepherd, we figured the people selling the pups just didnt want them any more. Wanted them to go to good homes, the registration papers were fake but we never had seen any before as all the previous pups were gifts or from free to good home, type multi colored pups with every one in the box was different sized, colored or longer fluffy or short fur.
If your ready interested in knowing your dogs inessential then I recommend a doggie DNA test kit. They are a tote spends compaired to peope ones (about $25 more) uness you get them on saw. You may be surprised as to what the dog is. He may be purebred and just looks geekey at this age too much puppy in him yet, or you may be correct in his mixture. There's no way to tell! for sure without the DNA test.
Have you tried contacting the people you bought him from?