1) You have sold property and made registered as well, in that you have not give time limit to pay amount back within so many days.
2) Now you have to talk with them politely and get done this agreement as soon as possible. Don't give possession of the property to them.
3) Start talking with them after every 3-4 days give 3 months chance otherwise send Notice to send complete the transaction in next 3 months or the deed will be treated as cancelled.
4) Now you may apply for cancellation of deed, consideration not received on this basis.