A few things come to my mind, did the agent put this in writing so you have some basis to claim fraud, was the appraiser acting as many do right now, conservatively and finally did the appraisal meet the value required by the offer and to obtain financing?
Did the agent makes a statement like, "This house is worth $1,000,000 but you can buy it for $500,000â€ in such a way that is provable or was it just hype? If the statement was in writing, you may have a cause to go after the agent for misrepresentation, but your contract with the seller may still be valid. To pursue your options here you will need to speak with an attorney.
If the appraiser came in at or slightly above the contract amount, this is common. After all of the problems with fraudulent mortgages & appraisals a few years ago, many appraisers are erring on the low side, especially if it meets the necessary contract requirements, rather than risk over valuing a home. You might try to speak directly with the appraiser and get your concerns addressed directly. There are rules about bedroom below grade they are required to follow. If the rooms have a closet and an accessible window, it may be a bedroom for your purposes, but not theirs.
Finally, if the appraisal came in at the offer price or above, your grievance again will be with the agent who claimed the higher amount. This may not invalidate your purchase contract. Again, you'll need to review your contract and possibly discuss your options with an attorney to be certain of your options.
I wish that we Realtors were able to control the Appraisals. That would be neat.
Question, was this the Listing Agent, or YOUR Agent?
Did you have them do a CMA before you made your offer?
Was your offer higher than the CMA?
You didn't say; did you get the house?
Good luck and may God bless