A lawyer has to provide the answer. I'm not a lawyer, but...
The agreement your mom signed did have "consideration": the real estate agent's fee. That's the contract between the agent and your mom. Now, a sales agreement between your mom and a buyer would need a price (or a clearly defined way to set a price at a future date).
And keep in mind that most agreements between an agent and a seller result in a sales amount different than that which might be specified. In today's market, it's often lower. It doesn't make the agreement between the agent and the seller invalid.
There are, however, other questions. Was your mom mentally competent to sign the document? Even if she is, in this situation--especially since she's granted power of attorney to someone to do her finances--it might make sense to include a contingency in the contract requiring the approval of her financial advisor for a sale to be transacted.
Check with an attorney. But--and, again, this isn't legal advice--ask the attorney whether additional protections, such as the sales agreement being contingent upon the approval of a designated financial advisor to your mother, should be incorporated into the agreement. And, if the agent is resistant to such a provision, speak to the agent's broker.
Hope that helps.