1. The steward was "unjust" for whatever reason he was being terminated by the master (i.e., "wasting his possessions").
2. The steward was "unjust" because his manipulation of the records was defrauding the master of property that rightly belonged to him.
3. The steward had been "unjust" in his prior dealings with the master's debtors -- perhaps by collecting more from them in the past than he was reporting to the master.
Item #2 is the most common interpretation of this parable, but I think this is the least likely of the three scenarios I've listed. The reason for this is three-fold: (A) because it's unlikely that Jesus Christ would tell a parable where someone was to be commended for theft; (B) someone in the master's position isn't likely to commend an employee who has stolen from him; and (C) because this makes not only the steward unjust, but the debtors as well (and yet they're never described in a way that makes them seem like co-conspirators in a crime).
Another important consideration comes after the parable as it was told in Luke 16:
Now the Pharisees, who were covetous, heard all these things: and they derided him.
Why would the Pharisees deride Jesus Christ over this particular parable?
I have no idea why. Explaining people’s actions is very difficult and most of the time I’m wrong.