1. Is there any clause in the said agreement mentioning that the agreement will be treated as cancelled unless the balance payment is made and the sale deed is registered for the said land within a specific period?
2. If no, then legally, you can neither forfeit the advance collected by you nor can cancel the said agreement without giving him any notice.
3. You should now send him a letter informing him that the said agreement will be treated as cancelled if the balance payment is not made within next 1 month and the sale deed is registered.
4. After the specific period of 1 month, send him another letter cancelling the said agreement and returning his advance amount.
5. If you do not manage to return his advance amount, he can easily get a stay from the Court restraining you in selling the said land to any other person and claim to register it at a later date as per his sweet will.
6. So, some how or other manage to deposit/remit the said advance amount to his bank account and then send him the letter cancelling the sale agreement.
7. This type of payment of advance without mentioning the forfeiture clause is a trap for unduly harassing the seller and keeping the agreement open for a long period of time.