Purchase agreement (APA/SPA)

The binding contract that governs the sale, an asset purchase agreement for most small deals or a stock purchase agreement, fixing the price, exactly what is bought, the seller's representations, and how a later dispute is settled.

This is where the letter of intent stops being a handshake and becomes enforceable; the indemnification and working-capital true-up buried in it decide who pays for a problem found after the money moves.