Software "ownership" is a bit more complex than Biopico's analysis would have you believe.
First, it is unclear whether "work for hire" by written agreement would apply in your case. Depending on how the contract is drafted, and on other factors, the customer may have a license in the software nonetheless.
Second, assuming you retain copyright authorship (which is different than "ownership"), you might have some practical rights in the software from a business perspective.
Finally, I'm not sure where Biopico gets the "labor and employment attorney" from, but he's wrong. Any qualified litigator can handle breach of contract matters, although you'll likely want one who's well versed in intellectual property as well.