Grave Scrabbler
Creature — Zombie
Madness 
(If you discard this card, discard it into exile. When you do, cast it for its madness cost or put it into your graveyard.)
When Grave Scrabbler enters the battlefield, if its madness cost was paid, you may return target creature card from a graveyard to its owner's hand.
Illustrated by
Commander 2019#117CommonEnglishLegality
Rulings
The second ability checks if Grave Scrabbler was cast as a result of its madness ability. It doesn’t matter if the specific cost wasn’t paid (for example, if the cost was reduced due to a effect like Cloud Key).
When Grave Scrabbler enters the battlefield, its ability triggers only if it was cast with madness. If it triggers, you must choose a target, but you don’t decide whether to actually return the card until the ability resolves.
You can return a card in a different player’s graveyard to that player’s hand.