If heron is at a battlefield and i use Retreat on it. do i get a discount on next card?

Yes — if Retreat is the first card you play that turn.

Heron's "if I'm at a battlefield" is checked the moment you play your first card. At that instant Heron is at a battlefield, so the trigger fires — Retreat resolving afterward and returning Heron to hand doesn't cancel the discount (same as the death/bounce rulings, FAQ 11792/11829). The discount is created once Heron's trigger resolves and applies to the next card you play this turn, after Retreat.

Caveat: if you'd already played a card earlier that turn while Heron wasn't at a battlefield, no trigger ever fires — Retreat later doesn't retroactively create it (FAQ 11906). Also note Retreat counts as your "first card," so the discount lands on the card after it.