If i have {centaur's harald} in hand, can i discard it (sac it) and pay the mana for the centaur. Or is discarding not the same as sacrifice ?
You can only sacrifice a permanent you control that is in play.
Discarding is putting a card in your hand into your graveyard
They are not the same.
What Sardok said. The summon centaur token ability of the herald can only be used when the herald is in play.
Summon the herald, use it as a chump blocker (vs a non-trample creature), then sac it for a 3/3 centaur token before damage is delt.