Closed-Door Negotiation's ability uses "trigger this effect again", meaning the effect will repeat until a player's opponent does not have more Æmber in their pool than the player does.
Example: Player 1 has 2 Æmber in their pool. Player 2 has 8 Æmber in their pool. Player 1 plays Closed-Door Negotiation and resolves the bonus icon, bringing Player 1's pool to 3 Æmber. Player 2 then draws a card due to the Play: ability, and Player 1 steals 1 Æmber, bringing their pool to 4. Since Player 2 still has more Æmber (7), the effect repeats. After this resolution, Player 2 has drawn a card and Player 1 has stolen another Æmber to bring their pool up to 5 Æmber, Player 2 has 6 Æmber in their pool. The effect repeats again, bringing Player 1's Æmber pool to 6. Player 2 no longer has more Æmber than Player 1, and so the effect's resolution is complete.
Example: Player 1 has 2 Æmber in their pool. Player 2 has 8 Æmber in their pool. Player 1 plays Closed-Door Negotiation and resolves the bonus icon, bringing Player 1's pool to 3 Æmber. Player 2 then draws a card due to the Play: ability, and Player 1 steals 1 Æmber, bringing their pool to 4. Since Player 2 still has more Æmber (7), the effect repeats. After this resolution, Player 2 has drawn a card and Player 1 has stolen another Æmber to bring their pool up to 5 Æmber, Player 2 has 6 Æmber in their pool. The effect repeats again, bringing Player 1's Æmber pool to 6. Player 2 no longer has more Æmber than Player 1, and so the effect's resolution is complete.