Haman called out his announcement that such shall be done for the man whom the king wishes to honor in the future tense. The easiest explanation may be that Achashverosh is rewarding Mordechai because he wants people to protect him in the case of future assassination plots. He is basically advertising, “You, too, can get this reward for protecting the king,” which is a statement which should logically be said in the future tense.
The Yalkut Gershoni quotes the Talmud (Bava Basra 12b) that since the ability to have prophecy no longer exists, prophetic insights have been given to fools, since people do not take them seriously. Haman was unwittingly saying that the reward they were all witnessing was indicative of Mordechai’s actual reward, which we will (iy”H) see in Esther 8:16.