Cash payment over the amount mentioned in sale deed

I entered into an agreement to sell for sale of one of my floors for Rs.25 Lakhs. The sale deed was registered showing sale consideration as Rs.10 Lakhs. Balance Rs.15 Lakhs was received in cash and agreement to sell executed earlier was torn. No photocopy was retained. I again entered into agreement to sell for sale for my another floor of the same area in the same built up property again for Rs.25 Lakhs. There are only 04 floors. The advance money of Rs.2.50 Lakhs for sale was taken in cash and a seprate receipt was executed apart from the agreement in which it was only written that Rs.2,50,000/- received in cash for sale of 2nd floor of the property. Now, on the basis of this receipt the purchaser has filed a suit for specific performance and is contending that the sale consideration for the 2nd floor was also agreed at Rs.10 Lakhs only and is relying upon the first sale deed to show that the value of the 1st floor was also only Rs.10 Lakhs. The agreement to sell mentioning Rs.25 Lakhs has been concealed by him neither do I have the photocopy of the same. How can i show that the sale consideration was Rs.25 Lakhs.