Sir agar apke father ka agreement to sale kua hua hai and amount receive hua hai to apko sale deed karni hogi nahe vo specific performance ki suit file kranga sale deed ka lia.

Ap suit contest kar skta hai per unka case strong hoga.

Since the sale is not complete as yet by making a registered sale deed or handing over the physical possession of it, you can cancel the same by sending them a cancellation letter and refunding the full money with interest.

Now the buyer if does not accept it then they have the option to file a suit for specific performance of contract to get the sale completed which would though would depend on the discretion of the court. 

It is necessary to peruse agreement for sake entered into between parties to advice 

there must be termination clause in agreement that agreement soap stand cancelled if full payment bot made within stipulated period 

You cannot cancel agreement unilaterally 

you can cancel agreement as per clauses in your agreement 

See you can file a cancellation but even possession on agreement to sale was given and they are using they are using it legally you can try contesting it by filling a suit of cancellation before the court.

You can't cancel it unilaterally if it's registered.  Otherwise you need to approach court

Sir see only chance is time as essence of agreement if time period of agreement is over or they failed to comply same then only at this stage you can go for cancellation on this ground.

Since you have objected to mutation of property in name of alleged purchasers they would need court orders to transfer property in their names 

ATS in no trasfer of property but terms and condition on which sale will proceed. Is there any time limit to make payment and registration of sale deed such date ? 

For any third party transaction done by them, themselves is liable. On the contrary, you can file FIR for forging property docuemtns or any threat.

They can seek civil remedy from court and for success of suit, they have to prove that have been ready and willing to perform his part of contract.

you can cancel the agreement to sell,  if the purchaser has failed to comply with the terms and condition of the agreement. The cancellation of an agreement to sell (sale agreement) depends on the terms mentioned in the agreement.. 

Since the sale agreement is valid only for a period of three years only, you as a legal heir  of your decreased father, can issue a notice to them communicating the information of cancellation of sale agreement which is barred by limitation and you may also inform that you are ready to return them the booking amount after deductions of administrative cost. 

This notice to be sent by registered post.

Let them take any legal action, you can challenge the same on merits before court of law,.

If you have to vacate them then you may have to adopt some strategies. You cannot say that you will not give them the booking amount especially when the proeprty is in their possession. 

It is better you settle the issue amicably instead of stretching the issue through a court case and losing money, time and energy by which there is no guarantee for a success.

If they do not have any rights to take possession of the property which has not yet been sold to them then how did you allow them to use the property till date.

Since there is a mistake  on your side in this connection, you may have to handle it patiently and carefully until you are vacating them from the property.

Once they vacate the property and handover vacant possession, you can dictate terms to return their money, so it is better to go for an amicable settlement with them to safeguard your interest in the property.