This is difficult and legal advice may be appropriate. It is important to put away the emotional for the cold legal viewpoint. My thinking is that the paper trail, what there is, will ultimately most likely be what a court would decide on (hope it never gets that far) and more importantly guide what is morally right. If you invoiced them for design time (sounds like you did), they should own the fruits of that time. If you invoiced them only for parts, and provided them the parts, then they have received what they paid for and you are done. The wording on those documents is important. Those unfamiliar with what's involved think design is free if they ask for some vaguely defined item and you make parts from their vague concept. Design is work and should be compensated for. Another factor in the decision (although not a legal one) is if there is any potential for future work. A good relationship leaves the door open for future development work with them. You have to price that development work understanding that that it probably will not lead to volume production. If that door is closed, there is less incentive for you to be a nice guy. They can always get their Chinese supplier to reverse engineer the parts. In that crystal clear 20-20 hindsight, clear definition at the outset prevents this from happening down the road.
Our government is screwing over and over again and killing manufacturing in the US by letting China print massive amounts of money to keep their currency cheap relative to the dollar and relative to its real value.