Partial sale of properties agreed in agreement (unregistered)

We entered into an unregistered agreement to sell two properties (one is owned by my father and brothers, another is owned by my mother. two properties are situated in different location) The buyer registered only one property which belonged to my father and brothers and mentioned that he would buy another property at later date. However, after an year and half he sent us notice asking us to register the second properties and we don't want to sell it to him for obvious reasons being the price, timing, situation etc. In the court notice, he has mentioned that we have failed to address his call to register the property by citing false reasons. As in many cases the the registered value for the first property is way less than the on mentioned in the agreement. The buyer claims that he has paid the cash on the date of registration which is not reported in the sale deed. What are the consequences of this situation? Although the unregistered agreement mentions the second property details and holds signature of my mother. The buyer claims that the amount mentioned in the agreement is inclusive of the second property and we are supposed to sell it to him. Please provide your valuable inputs on the above. Thanks in advance.