The appraisal is an opinion of value as mentioned in prior posts and is specific to that point in time / that transaction. The loan officer you worked with could have ordered another appraisal and sent it to a different lender if he/she felt it was a bad appraisal. The risk with this is that you have to pay for each appraisal and they're not cheap (approx $350).
The appraisal is not recorded anywhere so a potential buyer would have no idea that your home was appraised at a certain amount.
I hope this helps.
You may be unhappy with the results, but
The Banks are going to rely on the latest appraisal.
You may spend your money getting another appraisal, but
the bank does not have to accept it.
This may be another way of telling you that you offered too much for the house.
If the Seller is inflexible, you should look for another house.