Agreement of sale and general power of attorney with possession is valid or not in India?

Agreement of sale and general power of attorney with possession is valid or not in India? My paternal grandfather had 300 sq. yards of land in Hyderabad, and that was transferred to my mom's name (daughter-in-law). My uncle (mom's brother) has cheated us and sold out our land. Without knowing her, he has taken her registration office at closing time and signed the registration paper, and we don't even know at what price he has sold. Everything he said lied as per his pre-planned chakravyuh. Now I just want to know if "Agreement of sale and general power of attorney with possession is valid or not in India". The person who purchased from us (the vendee) has not done a sale deed, but he has done an AGPA with possession. The vendee sold out the land after six months to another person, but the vendee (who has purchased from us) did not take our permission or sign of my mother, as she is the principle holder. I have read some cases in which a clearly written principle needs to be signed while transferring property by vendee. So please provide your valuable guidance regarding Is an agreement of sale and general power of attorney with possession is valid or not in India? Is the principal holder's signature required while transferring property by vendee to others? Thanks in advance