Let's do this step by step.
Adept has Magic 5, picks 5 points worth of powers. He goes on a couple of runs and decides to get some cyber vision enhancements. This reduces his essence and drops him to Magic 4 with 4 power points, having to "lose" a power. If the power costs more than 1 PP (say 1.5 PP), the character now has Magic 4 with 3.5 PP spent. He could spend that .5 PP, but he would still need to learn the magic somehow. Or he could wait until he had enough karma to initiate or raise his Magic attribute, then re-take the power again.
Also, posts like this would get better/faster answers in the "Rules and such" topic for future reference.