Is your employer aware that he can "steal" back any ideas that other people get from your code? I mean, if they can read your work, so can you theirs. Maybe looking at how you can benefit from the situation would yield a better return of your investment than fearing how much you could lose.
[EDIT] Answer to Nick's comment:
Nothing gained and nothing lost. The customer has what he wants (and paid for it since he did the change himself). Since he doesn't release the change, it's as if it didn't happen for everyone else.
Now if the customer sells the software, they have to change the copyright notice (which is illegal, so you can sue and will win -> simple case).
If they don't change the copyright notice, the 2nd level customers will notice that the software comes from you original and wonder what is going on. Chances are that they will contact you and so you will learn about the reselling of your work.
Again we have two cases: The original customer sold only a few copies. That means they didn't make much money anyway, so why bother. Or they sold in volume. That means better chances for you to learn about what they do and do something about it.
But in the end, most companies try to comply to the law (once their reputation is ruined, it's much harder to do business). So they will not steal your work but work with you to improve it. So if you include the source (with a license that protects you from simple reselling), chances are that they will simply push back changes they made since that will make sure the change is in the next version and they don't have to maintain it. That's win-win: You get changes and they can make the change themselves if they really, desperately need it even if you're unwilling to include it in the official release.
