How to cancel agpa?

Am planning to buy a flat from Agreement of sale cum GPA holder. My sister has given the AGPA to him(3rd party) and He is planning to sell the flat. Am ready to take the flat from him. My sister has no objections on this. Please suggest me to go or not go with this purchase. OR How can we cancel the AGPA document from him(3rd party) ??

Talke a relinquiashmwent deed in your favor from your wi9ster and buy the property from AGPA.Your sister has cancellation power ofAGPA  of the sale and  GPA holder.

Thanks for info Usha ji. Plz suggest the process of cancellation of AGPA. And AGPA document, original sale dead docs are with the AGPA holder. How can we take it back? He will not support for the cancellation process. My sister can go and cancel the AGPA individually????

You can cancel GPA but not Agreement of Sale executed in it, with out the concent of the agreement holder

AGPA holder can sell the property individually? Banks will give the loan on this property?? (AGPA holder to 3rd person)