Is sale deed between a father and a son valid? Need to mention amount?

My Uncle has got registered GPA on one of the property to sell. Now , my uncle wants to give that property to his son. So, he is planning to register on his son . But, since it is a sale deed between father and son, should the sale cost be mentioned? OR can we go ahead with registration with out mentioning the consideration value. But, we will be paying the stamp and registration charges. Also, in the GPA it is not mentioned to GIFT. That's why they are planning to register.