Regarding sale agreement

I'm the seller of agricultural land. I have signed a sale agreement and taken 3lakh as advance. It was in 2016 and it was a 1 year agreement. I knew the buyer from past 10 years. After that the buyer was not interested and we let it go and money I told I would give it in installments monthly 10k. In 2018 the buyer died and their family member is trying to buy the land. And I'm not interested because the value have increased. Now she has sent me notice from advocate to fulfill the deed as stated in the papers. And she states in the notice that I have taken extra 9 lakh from the buyer which I didn't take. Sale deed details. I have stated that I will fulfill deed if not after 1 year they can take legal action in law against me In agreement there is no statement of the buyer wife I have given 10k from 2017 till 2020 Jan which is more than what I have taken. But I don't have any kind of receipt for that and she has stated I have taken 9lakh extra from the buyer which I didn't and there is no receipt for that also. What steps should I take