Black & White

Hello Sir/Madam, A sale deed, registered at the circle value, benefits a person, getting concession in stamp duty & Registration fees, but by which document additional amount paid in cash above the agreement value not entered in the sale deed while registration, be secured, in short ( black money), if any dispute with consideration (paid in cash) arise in near future so that the buyer is indemnified with total amount paid i. e agreement value and additional cash so given?