cancellation of unregistered bond sale agreement

You will have to issue a notice qua termination of subject contract (agreement to sell) for non-performance of contractual obligation on the part of vendee (purchaser).

You can very well rink up the prospective Buyer, with whom you entered into an agreement and discuss the matter. After all, this lockdown was not anticipated when the agreement was drawn and signed. I can say it is like Force Majeure (Act of God), both of you can very well come together over the phone (never to venture out till lockdown is lifted) and can have a new schedule. 

Moreover, from what you queried, the agreement appears to be unregistered and I do not know whether proper stamp duty is paid or just written on white paper. 

Have you contacted the buyer - if so, what is his response?

Although in due compliance of the terms, the Sale Agreement has not been registered during the stipulated period of 60 days from the date of execution of that unregistered Agreement,if the Seller tries to cancel the Sale Agreement on that groud,then that conduct of the seller  is liable to be challenged on the ground of Clause force majure because it is necessary to exclude the Government directed period of lock down( to maintain social distancing) while calculating the 60 days time period.After withdrawal of lock down,it is necessary on your part to give notice to that buyer in this regard,specifying the date for completion of the performance of Sale Agreement.Please take note that I am giving this opinion without perusing your document so for detailed opinion,it is necessary to see the document as a whole....Sinjari Banerjee,Advocate from Kolkata,9830019661 ,9830011829.

 Through EMAIL or message,you can ask the buyer for online transfer of the balance amount to your bank amount immediately and also notify to do the needful for E REGISTRATION OF THE Sale Deed.