MC lacks wisdom, it seems. First slave cheats on his ambitions due to his inability to craft a better narrative than a lie to get the slave conformant to the goal. Not only that, as another mentioned, a lie that appeals to the other's goals directly, instead a servitude or alignment.
The moment the slave was so happy, I would have questioned the interpretation it had of the conditions it has. Maybe kill the slave or torture it for months to make up for the transgression. Will MC redeem himself or get pussywhipped?