A prospective buyer is not required to inform the seller about the market value of the property at which it can be sold
So it cannot be said that the buyer cheated your father
The seller was required to be vigilant and made his own independent inquiries regarding the value of the property rather than depending on the agent or his buyer
Seller has entered into an agreement with the buyer and also received 25% sale price
Now the seller cannot be heard to say that there was some foul play
It was the seller's mistake negligence and ignorance for which the buyer cannot be faulted
Hence there is no ground for rescinding the contract
The only option would be to talk with the buyer and renegotiate on the price
The buyer by entering into some agreement with a third party would not mean that he is selling your property
An agreement to sell does not create any legal title or ownership right in favour of the buyer unless a sale deed is registered
However in the event the buyer defaults in his obligations towards the third party then the latter can sue the buyer and may even go for an injunctive suit against him thereby affecting your property. Since if the third party succeeds and an injunction is granted against your property when it's not even finally sold to the buyer, your property would be locked in dispute. So you can use this as a tool and ground for terminating the contract with the buyer and on that pretext try to renegotiate on the sale price on its higher side