Stamp expenses and section 43CA

The assessee is a real estate developer. He sold plots not below the guideline rates. As per sale deeds the assessee borne the expenses on stamp duty and registration fee. The assessing officer is of the view that in the form of free registration facility the assessee has offered a discount whereby the consideration received is not equal to the guideline value and therefore the expenses borne by assessee on registration fee and stamp duty is the difference between actual consideration and guideline value and he proposes to add such difference u/s 43CA. Please guide. Thanks