You definitely should be contacting the seller about this, regardless. Whether you can give the dog back is largely dependent on your contract with the seller, and what guarantees (if any) the seller made regarding temperament. That said, and if I had to guess, your seller did very little or no socialization with this dog. That should have been question 1 when interviewing the seller. If you don't have a contract, the behavior can be modified, but a certified professional would need to assess whether this dog under your guardianship is a candidate for behavior mod.