Do you have an agent? That is who knows this contract and should be advising you. I don't know MD laws.
The previous poster must be referring to how their contract works. I, too, am in VA and will tell you that if you don't have an appraisal contingency, and the buyer tries to back out because of it, they would be likely considered in default, and an attorney could be brought in. You could potentially sue for damages - which might be the sales price. The original poster also is likely using a different contract, because in the DC metro area we all use the same Regional Contract - so the one I use may be the same one you are using. And, you very definitely chose if there is an appraisal contingency or not.
Of course, if they have a financing contingency and now can't get financing because of the appraisal... well, it's a problem; regardless of whether or not are in breach of the contract. Remember, even if you take action and win, none of this happens quickly, unfortunately.
Appraisals are a common issue today - in fact, I believe it is a sign of a changing market. We always have problems with appraisals at the bottom of the market. Buyers are valuing property higher than appraisers support. Overall, its a positive. In your case, and in individual cases, it's an issue - and a very big one.
But, before you get desperate and start calling attorneys, look at the appraisal - maybe the appraiser got it wrong. I have challenged appraisals and won simply by supplying another comp or more information; or by correcting factual mistakes about the subject property. If that fails, then suggest another appraisal, maybe offer to pay for it. They may have to switch lenders, and it could cause a closing delay. But, if it means another $11K in your pocket, it's worth it.
I am throwing thoughts and ideas at you rather than solutions, because, again, your agent needs to help you find the solution. It's just too difficult from over here on the other side of the river. If you don't have an agent, please talk to an attorney to consider your options.
But, remember, the most successful negotiations are those that keep in mind the common goal. They want to buy your house. You want to sell it to them. Success comes when you work to resolve the issue together.
Best of luck.