Registered Agreement and POA

In the year 2009 my mother gave Power of Attorney(POA) of a land to a person who was close to the family for selling the property. In 2017 my mother cancelled the POA since the person did not sell the land and also did not contact us regarding the land. Our multiple tries to reach out to him also did not yield any result as he never responded to our calls. Upon cancelling the POA , My mother received a notice from another person who we later found it the own brother of the person to whom my mother had given the POA. As per the notice sent by him , my mother had done a registered agreement with him after receiving a specific amount for the land in cash and post that my mother has not taken any step towards registering the land to him. This was a total surprise for us as we did not know this person and he was also not available during signing the POA. Upon verification we found that there is a registered agreement done with the signature of my mother on the same day when the POA was signed. Need your valuable advise to understand how to proceed. Is the agreement valid since my mother who is the owner of the land is totally unaware of signing any such agreement.