You probably should have asked your initial question re this issue in the intellectual property category -- because the other two answers you received in the employment law categories are generally inapplicable to your issue.
case, that you provide stands for the proposition that an author of a creative work (such as a software application) may grant an implied nonexclusive license when (1) the licensee requests the creation of a work, (2) the creator (the licensor) makes that particular work and delivers it to the licensee who requested it, and (3) the licensor intends that the licensee-requestor copy and distribute his work. IAE, INC. v. Shaver, 74 F. 3d 768, 776 (U.S. 7th Cir. 1996).
On your facts, you were hired to produce a software app; and you made and delivered the work. The question is: did you intend that the purchaser have the right to copy and distribute your work -- and the corollary issue, which is outside the scope of the Shaver decision: did you grant a license to the purchaser to create derivative works
from your work?
These questions render the attorney's citation to Shaver
, interesting, but hardly determinative of the entire issue. This is, however, the sort of case that could require a fair amount of litigation to decide, because the purpose of your work, and the purchaser's intended use could weigh heavily in whether or not the purchaser has a license to modify and distribute your software application.
There is also an employment law issue, which I do not believe either of the employment law contributors with whom you previously corresponded considered.
Act (and consequently, any intellectual property lawyer) does not
view an employer-employee relationship in the same manner as does an employment rights attorney.
The fact that parties to a contract may have agreed to an independent contractor agreement between them, does not necessarily mean that the relationship, for purposes of copyright law, was in fact an independent contractor agreement. The court will look at the totality of circumstances to see whether or not you were actually a common law employee, despite your independent contractor agreement -- and if the court were to decide that you were an employee, then the purchaser would actually own the copyright, despite the absence of any written agreement.
Once again, this is all highly fact dependent litigation material. Which means that neither side of your dispute has a lock on how the issue is likely to turn out.
What I can say with some certainty, is that the litigation would be very
expensive. I doubt that a competent intellectual property lawyer would take this case for less than $10,000 up front, and if the matter goes to trial, I could see it costing in excess of $25,000 per side. Hopefully, this would give your opponent some incentive to settle.
In my opinion, if you want to push this matter forward, you will need to hire an intellectual property lawyer to send a demand letter. Otherwise, your opponent will probably view your demands as not sufficiently serious -- especially as this sort of claim can only be made in Federal District Court -- i.e., there is no inexpensive means of resolving the matter, unless you each agree to binding arbitration.
Hope this helps.