Agreement to sale to be cancelled

My father and elder brother had decided to sale some property (1.5 acres of agriculture land) however the next party did the agreement to sale on my father’s name when my brother was out of town and they said that this agreement to sale will just be notarized and give into bank for their bank loans to complete this deal; but later they approached my brother and ask him for sale deed of 1.5 acres of his land which was decided as per commitment of both parties earlier but now the situation is - my father has signed the agreement to sale of 1.5 acres of his land and sale deed has happened with my brother on his 1.5 acres, we approached that party and asked them to return that agreement to sale which was done on father’s name however they are not returning it and giving some excuses day by day, sometimes they are saying we have shredded it, sometimes saying it is in bank for that loan they have taken. Queries are - how we can cancel my father’s agreement to sale? If that party has deposited it into bank then does my father has right to ask bank to show him the same so that we can verify what is written in it? What if my father makes gift deed of that land on my 2 sister’s names (this is what we are thinking)? Please advice.