Forcing the purchaser to pay more than the agreed amount

I bought a property from friend. We agreed to close our deal a year later. I paid him 40,0000 $ advanced. We signed a legal binding agreement. All good. After one year I asked to close our deal and he started asking more money. Property value gone up. I told him no, you are breaking your promise and agreement. He put pressure on me. Literally black mailing me. Otherwise I go to court and suffer more financial hardship. I surrender and promise verbally to pay 40k extra by taking a loan and paying him off. Now the deal is closed and I feel I should not give extra money that I don't even have but I feel bad that I am breaking my promise. Also I felt I was cornered. Please help me what should I do?