Agriculture land sold at guidance value

I sold 10 acres of my agriculture land to my mother-in-law. The market price is 30 Lakhs but guidance value is only 10 Lakhs. So we made a sale deed for 10 Lakhs for which she gave me a cheque for 10 Lakhs. Now that registration is done when I demand the remaining 20 Lakhs she is no mood to give that. Since it is within relation we didn't made any agreement saying the deal is for 30 Lakhs. What is the winning probability should is sue her?