It is best to read and understand what you signed. That is your agent's job.
Regarding reducing the commission, it is not up to the agent, but up to the BROKER.
Good luck on your house appraising at purchase price.
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If you are seller then your listing agreement terms are what cover your duties to your agents and the contract terms for the sale will effect the sale. Under appraising alone may not be grounds for reducing your sale price. Speak with your attorney. If however it does and you become a short sale, negotiations may begin with Realtors, your lender and your buyer. Until you check with all these parties you may not know all your options. (lender could forgive some of your debt or buyer could up their down payment if needed) Buyer could also try getting another appraisal as these are opinions of value and subject to variance.