Validity of sale deed of agricultural land

My grand father Mr Mapari have sold his agricultural land in 1942 to Mr. Vikhe returnable basis and my grand father release that land in 1952 as per sale deed clause. After 1952 to 1972, my Grandfathers name shown on the 7/12 extract. After demises ofm my grandfather land is transfered on the name of my father (Mr Laxman. Till 1981 land was in the name of my father and land in the custody of my father. In 1981 suddenly land is occupied by our some relatives (Son of Mr. Vithoba) and they have shown sale deed dated 1952. Relatives are saying that their father Mr. Vithoba have purchased this land just after two month when our grand father released this land from Mr. Vikhe. Now our relatives name shows on 7/12 extract. We have checked fer in tehsil, there is no approval remark to that fer number. Now our question is that why they occupied land in 1981 if they purchased in 1952? 29 YEAR OLD SALE DEED IS VALID IN 1981? Why my father have not recieved any notice before changing names on 7/12? We feel something wrong in this matter, so need your guidance.