The first 2 wishes of the Orc king to Malor were the following:
1. Build a new fortress over the tomb of Duke Ulderek (an orc leader killed by the dwarves).
This wish was fulfilled (he built the Orc Fortress).
2. The Orc King's second wish was to become immortal.
This wish was also fulfilled, he's immortal.
3. The third wish was to father more fertile children than any other orc before.
This wish was also fulfilled:
By transforming the Orc King into a type of "slime", it makes him easily reproducible without much effort (just like slimes which multiply quickly). At the same time, their children are also slimes and therefore, just as fertile as he wished.
Malor never cheated the Orc King, rather he lacked being more specific in his wishes...