The agreement for sale is valid for three years. If there is a negative clause in the agreement, say, the buyer has to register the property within three months’, then, the limitation is extended by such period.
It is in your interest you comply with the following in spite of negative clauses favoring you found in the agreement:
If sale is not registered, then, you have to issue notice of revocation and cancellation of agreement even though the agreement is cancelled by flux of time. Validly cancel the agreement. It is always advisable to do this.
It all depends on the clauses framed in the agreement.
It is always good to refund the advance taken.
There are many citations from various courts of law. I think it is not required to be shared at this juncture.