Strange animated button

Hello everyone

Perhapse this one is too much about details but anyway - hope the flashmasters will give me an idea how to solve this problem, cause I couldnt find it anywhere on net.

I will use an example to make it more clear. Lets say we have a button that fades in from Alpha 0% to Alpha 100% when rolling over it and that fades out when rolling out from it (Alpha 100% - Alpha 0%). All the tutorials used method where when rolling out it played “fade out” tweening from the very beginning. How to make button that for example when rolling out when the fade in effect is not finished yet will fade out from the position where it stops fading in? Desirable effect is when rolling out at Alpha 50% would fade out from 50% not 100%

Hope its clear enough
Thanx in advance