if heron is on a bf, i cast discipline, my opp hard bargains it, i defy the bargain my first resolved card is defy, but the next card after is discipline which i already paid for to play do what happens to the trigger for heron

Your Heron trigger fires on Defy — but Discipline does not get the discount, so it resolves at the price you already paid.

Here's the order: Defy resolves first (countering Hard Bargain), and since it's your first card to complete resolution, Heron's trigger fires right then (Rule 419.4.a). The trigger goes on the Chain above Discipline and creates the "[2][A][A] off your next card" discount. But the discount applies to the next card you play after the ability resolves — Discipline was already on the Chain and had its cost paid before the discount existed, so it's unaffected (FAQ #11762).

Net result: Hard Bargain is countered, Discipline resolves at full price, and the discount is saved for the very next card you play after the Chain clears this turn.