I don't know the rules of using scrolls, but he used a scroll, not a spell slot.
It differs slightly between systems, but in DnD 5e, you can only cast spells from scrolls if they're on your class spell list, and if they're higher level than you can normally cast, you also need an ability check.
The system here is some kind of homebrew, though, since it doesn't exactly match up against any specific system (though I use DnD 5e as a comparison, since it's close).
My problem with this one was that Haste was listed as a level 2 spells, which would be wrong either as a spell slot or as a scroll.