According to Shelom Esther, by appointing Mordechai in charge of Haman’s property, Esther was in effect making Haman’s family into Mordechai’s slaves.
R’ Avraham Mordechai and R’ Dovid Feinstein both quote the Talmudic (Pesachim 88b) dictum that what belongs to the slave is really the master’s. Therefore, Esther’s action returned Mordechai’s property back to him.
The M’nos HaLevi and R’ Shmuel de Ozeida note that Esther could not have given this to Mordechai outright because it was from Achashverosh (see Esther 8:1). It could seem like a slight to Achashverosh’s honor if she were to re-gift Haman’s property directly, so she appointed Mordechai in charge of it, instead.
The Sfas Emes interprets Haman’s estate as the other-worldly powers he amassed. At this point, Mordechai became the master of these. Perhaps this black magic can best be described as the power to change the spiritual world. Just as H-Shem placed us into a physical world where we can do such things as control electrical currents with switches and harness the wind with sails, He created our souls in a spiritual world which we can also affect if we want to.
The Maharal notes that this act points to a major theme throughout the entire Megillas Esther: that absolutely every single thing Haman attempted to do was turned around on him.