simple physics; mice are way stronger AND way faster than arthropods.
i speculate it would start by biting/ripping out the scorpion's claws, as the scorpion would raise its claws up to defend itself. The scorpion might even try to pinch back at the mouse, but its grip is not strong enough to hold a mouse's limb for any duration, so the mouse would almost instantly wriggle its leg free with only a minor flesh wound. however then the scorpion would reflexively sting the mouse upon first contact. Most scorpions' venom+dosage isn't nearly potent enough to kill the mouse right away, and it might not even be enough to kill a mouse, period. So the mouse would turn from tearing/biting out the claws, to tearing/biting out the stinger instead. That would take about 10 seconds, maybe 30 seconds tops. Then it resumes tearing out the claws, which doesn't take very long either. Then the mouse has a completely helpless scorpion with which to devour at its leisure. Though, depending on how much venom was injected (if any was injected to being with, as its possible the mouse could avoid the sting entirely), the mouse might be feeling pretty shitty or even dying after the meal.
its also possible that the mouse may choose to attack the stinger first, if typical mice have such knowledge/instincts available to them. There are alot of desert rodents that prey upon scorpions and other arthropods, and these guys could eat a scorpion without getting stung at all.