Rpg Stats - Temporary Changes, Harder Than I Realised!?

Rpg Stats - Temporary Changes, Harder Than I Realised!?

I'm writing a turn-based RPG, and have a small coding dilemma about stats. Specifically, it's about when you need to make temporary changes. Say the player has a speed which is currently 50, and the spell "slow" takes 10 off the speed. So it's reduced to 40, and in a few turns it has 10 added back so it's back at 50. Fine, cool with that.

Now, suppose the speed is 5. I take the 10 off, but aha, can't be < 0, so I set it to 0. A few turns later I restore it, but now it goes back to 10, which is clearly wrong and an exploitable side-effect, so I don't want that to happen.

I have 2 choices as I see it. 

1. Allow the speed to be negative, but whenever I use it, I clamp it to the range [0,100] (say). Note that the same goes for if the speed is 95, I allow it to go to 105 if the "fast" spell is used. I need to be very careful when coding this, because if I fail to clamp, things will go quite wrong and it will be a bit unpleasant to track down.

2. Create an undo record that says "only increase by 5 when you put it back". This requires an architecture for storing the changes, but also needs quite a lot of care to ensure consistency and I am not 100% convinced it can be made to be consistent because other things can be messing with the stats inbetween the change and the undo. I rejected the "store the actual speed it goes back to" option for this very reason.

So: am I missing other possibilities, or if not, which of the two appeals and why?