According to R’ Eliezer of Worms, the verse use the word “parashas” (“chapter”) to describe Haman’s meeting with Achashverosh in order to emphasize that this event was not just talk – a financial exchange took place, giving the event legal significance and legally binding consequences.
In fact, as the Divrei Shaul points out, Mordechai was communicating to Esther the fact that Achashverosh could not be easily bought off, since Haman had already given/ offered him money (see above 3:11).
As R’ Dovid Feinstein notes, Achashverosh’s refusal to accept the bribe only stresses the ferocity of his hate for the Jews, making this a very serious threat to Jewish existence, indeed. Convincing Achashverosh to change his mind would require nothing short of a miracle.
Parenthetically, According to Yaaros Dvash, the fact that Achashverosh refused the money was covered up by Haman in an attempt to deny people the opportunity to intercede on behalf of the Jews.
The Chasam Sofer writes that Mordechai’s giving the details of this entire episode here served a vital purpose later. In fact, Esther uses this event in detail to convince the king to save the Jews from Haman’s decree. In the Chasam Sofer’s view of the events, Achashverosh refused Haman’s money (see 3:11 above) because he reasoned that killing such a people was a worthwhile responsibility of his, and taking payment for this would be unethical. In Achashverosh’s mind, Haman’s offer and and his refusal were secret. Since Mordechai’s knowledge of this came through a Ruach HaKodesh-like dream, Mordechai kept the information under wraps to be used later, if necessary. Once Esther tells Achashverosh that her people had been “sold” (see 7:4 below), Achashverosh begins to suspect that Haman had libelously spread the rumor that he had, indeed, accepted Haman’s payment. Therefore, he responds by asking who would do such a thing (see 7:5 below). It seems improbable that Achashverosh had forgotten the entire incident, so he is asking who would spread such a rumor.
The Brisker Rav interprets the word “parasha” as being related to “lihafreesh” (“to set aside”). In his view, Mordechai was informing Esther of the money that was set aside, or designated for the purchase of killing out the Jews. Such money was legally binding, and eventually, Achashverosh’s only way out of the deal would have be to kill the “buyer” – Haman.
Alshich uses Rashi’s seemingly simple explanation that “parsha” means explanation to mean that Mordechai related all of the details of Haman’s and Achashverosh’s meeting, including the mystical interpretations for the reasons Haman had to offer 10,000 loaves of silver.
Finally, the Sfas Emes translates “parashas” to mean “sum,” emphasizing Haman’s generosity in contributing towards the kingdom. In view of the concept of “zeh l’umas zeh,” the Jews need to be generous with charity in order to counterbalance the generosity of our enemies.