pin in leg

Don't just speak to the vet that did the surgery - insist that he give you the dog's records (if he hasn't already) & explain to you exactly WHY the dog has a pin in his leg. He had to have made a diagnosis & had some reason for major surgery on a little dog.
   
Even if you have to pay a copying fee, get the records. Have your present vet demand the records, too.
   
And have your present vet explain precisely why the dog has to have the pin taken out, before you schedule major surgery - again.

And OF COURSE take him to your vet! He's your dog, now.

Yes, it is your dog now. But the records belong to the vet and he should NOT release them to you! The dog was not yours when he treated it, so you have to have the previous owner get the records for you. Also, if the bill has not been paid in full, the vet is under no obligation to release the records to anyone.
   
It is not unocommon for the pins to be removed when an animal has a fracture repair - depending on the location of the fracture.