Just take a deep breath and calm down!
You made a mistake, there are plenty of people on this forum that have made mistakes just like you.
It does not mean that you are getting a puppy that is going to have a genetic defect. You will fall in love with him, you are
not going to have to rehad a puppy like an adult and you will just need to take one step at a time.
When does he come home to you? I do suggest that you have the testing done that Ladymom suggested but please relax. You have already made your decision that you are not going to back out of the sale so you will just need to take it as it comes. If you need any help just ask and there are plenty of people who have all types of information that is very helpful with raising a happy and health puppy