Esther summoned Hasach, one of the eunuchs whom the king had appointed to serve her, and sent him to Mordechai to learn the events. Hasach went out to Mordecai in the city square in front of the palace gate; and Mordechai told him all that had happened to him, and all about the money that Haman had offered to pay into the royal treasury for the destruction of the Jews. He also gave him the written text of the law that had been proclaimed in Shushan for their destruction. He told him to show it to Esther and inform her, and charge her to go to the king and to appeal to him and to plead with him for her people. (Esther 4:5-8).
Why was it important for Mordecai to tell Esther about the money Haman offered the king to wage war on the Jews? After all, the king was aware of Human’s offer directly from Haman himself.
It may be to show Esther that Haman was on a lower level than Achashverosh, and that’s why Haman sought to pay to destroy the Jews. This fits in with Mordechai’s plan, because it revolves around Esther turning Achashverosh against Haman. Mordechai urged Esther to reveal to the king Haman’s anti-Semitism (he’d even pay large sums to kill Jews), exposing that Haman had no real loyalty to the king; he operated personally. This could motivate the king to turn against Haman and save the Jews. And although the king knew of the monetary offer, Mordechai might have also intended to fortify Esther emotionally against Haman which cold be a daunting task.