Proerty documents. Revocation

I am buying a flat in karachi. The owners are the husband and wife, who are alive and living in that flat. These parents have given the attorney to their son and wife of their son who are living in canada. This attorney has been registered in canada. Can i purchase the flat with revocation of attorney afterwards? Or i should ask them to revocate first then i will make the sale deed with the parents (owners) of that flat?