Either way, I would let your lender look at the appraisal so they can tell you all the reasons that value is not supported. His comps are probably all adjusted to the wazoo.
By chance they are not the lender can not use that specific appraisal but they can send the comps to the original appraiser for additional consideration.
Honestly, how did it get this far? Once the initial appraisal was completed by the lender, you still had an option to cancel escrow. If your loan/appraisal contingency does not extend this far, then you may be stuck with the agreed upon price. If you can't perform, then seller may cancel and pocket your deposit. This depends on how the contract was initially written, tho.
Good luck and next time get a better agent that can advise you on these matters. Agents that just tell you to "buy buy buy" or "sell sell sell" without explaining the consequences appropriately should not be your agent!
Jason Stevens, GRI,ABR
Kevin Flynn Realty, Inc.