Dispute in selling a property purchasor lingering for 8 years for payment.

My father had an unregistered agreement with a purchasor for selling a flat. the agreement was done on march 2010. from then onwards the purshasor is paying in installments and till date he has paid 80% of the agreed amount. however my father died in august 2017. after that we had not accepted any money from purchasor. our flat was ready by 2014. but there was delay in municipality paper works from our side. registration has not yet been done with the purchasor. we have requested for a settlement and we agreed to re-pay the paid amount till date to the purchasor. but the purchasor is not willing to take the money back. now i, my mother and sister are legal hier for that property but do not have any will from my father. my question is 1. is the agreement valid if no date mentioned in it for handing over the flat? 2. if i re-pay what will be the minimum interest as per law? 3. If an agreement for selling a property is unregistered, what is the period for its validity? 4. what should be my immediate next step? Regards