DIFFERENCE IN SALE AGREEMENT/SALE DEED

My grandmother (mothers mother)nacquired a property in Cheñnai in 1936 My grandmother had only one child ie my mother withbrothers/sister's to my mother. My grandmother passed away in 1991 and my mother bequeathed property in her sole name in 1991 My fatherpassedawayin2007 My motherhas fourchildrenthreedaughtersandmyself her onlyson. My mothermadesettlement deedin2008 in my favour for the property but held ownership rights and my mother settled ownership rights in my favour in August with registration of settlement deed as she is very old. IS noc from mysistersmust?n2 Buyer wants to enter into sale agreement and wants to showslightly higherpricethan agreed price orally for getting higher bank loan. He says sale deed will show only the original agreed price and seller wont be impacted at all on account of showing higher sale amount. Pl let me know if this is legally valid or I should not agree to his conditions and ask him to stick only to oral agreed amount only in sale agreement.