Agreement on a Stamp Paper by both parties with witnesses

My friend (A) is the actual owner of a Plot. He purchased from B who done the transaction thru notarised document 25 years back. B expired 18 years back. My friend A paying the Property Tax & Electricity Bills and enjoying the possession. My friend wants to sale the said property to C without any written agreement. just verbally only. C paid 14.5 Lakhs to my friend A in this regard in phased manner. Absolutely there is no written agreement. Now my friend A decided that not to sale the said property to C. He clearly told C, not willing to sale at this juncture. C annoyed and asked about the Advance Amount which he paid to my friend A i.e. Rs.14,50,000/- + Compensation Rs.7,00,000/-. My friend A issued the Cheques for the total amount i.e Rs.21,50,000/- to C. with ill intention C received the Cheques and claiming more amount i.e. Rs.25,00,000/- from A. on 30th December in the presence of eleders as witnessed they made an agreement on a Stamp Paper saying that A has to pay Rs.25,00,000/- on or before 30.01.2021 to C, if not paid the said amount by A, A has to registered the plot to C without any further delay on 02.02.2021. Now to drag on the issue C went to B's legal heirs and pressurising A not to sale the property and threatening him the issue. The main reason is if A failed to pay the said amount to C, A has to loose the property to C. That's Why C is threatening to A in various angels. Most important thing is C is tenant in the A's Property and claiming that he wont vacate the House until unless to pay the said amount or register the vacant plot on his C's name. A is an Ex-Army Person. C is a Railway Employee. Humble requesting your goodselves, kindly provide a suitable solutions please.