1. The sale deed once executed and registered can not be cancelled in any circumstances.
2.only if the sale deed is done by force, fraud , coercion or misrepresentation to the material fact then the court can on proof of these allegations can declare the deed as null and void.
3.So in your case nothing will happen as you did not sold the proeprty under force nor fraud was perpretaed on the purchaser.
4. So do not worry the purchaser can do nothing.