I see a problem with this, but it's not a big deal. It's fairly clear that a convo like this is just fooling around in play and the Dom wants to adminster some play punishment. No problem on that level.
The problem is that a Dom's should not fool about with instructions. The Dom said from now on 2+2=5 so it does not matter what the calculator says, the Dom's instructions take precedence.
It's bad leadership and (wo)man management to play around with instructions. The downside is if you make a habit of this when you give a real instruction the subbie can justifiably say "I thought you didn't mean it. I thought you were joking around like you did before". There's plenty of petty faults you can find to use as an excuse for punishment without messing about with instructions.