Movement Speed

I made a really cool charge spell that disables user control but keeps the camera focused on the hero until death or contact with target but my only problem is the movement speed. in my trigger i adjust the MS to current MS +500 and after contact or death occurs i deduct current MS by 500. why then does it make the MS extremely slow as if deducting 1000 instead of just returning to the original MS. i tried storing MS as a variable too but wasn't sure about that.

The maximum movement speed that WC3 can handle is something like 522. So your adding +500 but its stopping at 522 then your removing 500 which makes it have a movement speed of 22.

EDIT: I do believe theres a unit state you can use to get the default movement speed of a unit...