Matthew 24:45 uses the Greek verb κατέστησεν to describe Jesus' appointment of the faithful and discreet slave. But καταστήσει appears in Luke 12:42.
So the Apostle Matthew employs the aorist indicative tense while Luke conscripts the future καταστήσει. Why is there a difference in tenses between the two Gospels? Is the difference substantive?
Regarding the aorist, modern studies in New Testament Greek now tell us that the aorist does not necessarily signify that an action is performed once for all time. The punctiliar nature of an act is derived from the context of a verse and not the aorist tense alone. The aorist is an example of what grammarians and linguists call, perfective aspect. What this means is that the writer (in this case, Matthew) evidently visualizes and subsequently presents the action described by the verb (aorist form) as an undivided whole, without much concern for the progression of the action or its telicity.
A reader can discern whether an action delineated by the aorist is punctiliar or otherwise by taking note of contextual or other linguistic features (also known as affected vs. unaffected meaning). As a side point, some grammarians classify the aorist as external rather than perfective aspect.
Daniel B. Wallace ("Greek Grammar Beyond the Basics," page 501) also classifies the future tense as perfective or "external" (his terminology) aspect. If this is the case, then it would apparently mean that there is no theological significance in Luke employing the future instead of the aorist form of the verb. I think the result is the same, regardless of which morphological formation the writer used. Both writers portray Jesus as appointing the faithful and discreet slave, then augmenting his authority once the Master arrives (see Luke 12:44).