Legally, the issue would involve intent. If Mirco "borrowed" a playfield and simply forgot to return it, it isn't theft. If he intended to never return the item despite promising to do so, it would be legally considered theft.
Of course now that he is on the forum here saying he will return said items, if he does not then there is a bigger issue.
I just don't understand the business sense of legally threatening a customer who allowed you to scan their NOS playfield in order for you to sell repro playfields to others. I would consider the repro playfield as payment for allowing the use of my NOS playfield for scanning. No way anyone would agree to trade an NOS playfield for a repro.
If your customer is mad, just send them their property. Sheesh.