Non Registered sale agreement

My father & sis brother has an ancestral property. The property is in his mother’s name & she is no more. My father -in-law & his brother are the legal heirs of the property. The property is undivided among the brothers.   Both the brothers decided to sell the property and had an agreement with a buyer on 08th Jan’2021 for 100 days and received 5% of value of the property as advance. It is not a registered sale agreement. Buyer have not registered the property till date stating various reasons. He made multiple promises on registration dates (atleast 3 times b/w Jan'22 to Apr'22) and failed to register the property.   In the sale agreement (prepared by buyer) it is mentioned that if seller is not agreeing to register the property in 100 days, he can go to court and by paying the agreement value he will force the seller to register the property on his name or to a person nominated by him. However, it is not mentioned in the agreement, if buyer failed to register the property in 100 what will happen to the sale agreement ? Original sale agreement is with the buyer.   My queries are the following : Is the sale agreement still valid (after 460 days) ? Is it required to for a seller to send a notice to buyer for cancellation of non-registered sale agreement ? Do seller need to return the advance to buyer as per law as he failed to register the property in stipulated time as per agreement ? Can buyer file a case against a seller (or) force seller to register the property, if seller deny to come for registration ? Can one of the seller (my father-in-law) gift the property to his only daughter before dividing the property with his brother?