I'm not an expert on FDA regulations so I can't really comment on those, but seeing as nobody is replying, I'll comment on an EU perspective.
From the perspective of placing on the market, you cannot encourage or suggest off label use, this is also the case in the US.
If you are selling a product to someone that you know is going to use it off label without any encouragement the EU is less clear, but I would be inclined to refuse the sale because you are "aware" that it will be misused and therefore you could technically be considered as marketing it for off-label use.
If you have already sold it (which sounds like your case) you cannot stop them using it off label, you did not ignore the regulations and market off label so you are clear in that sense - it was their own decision. In this case I would probably document that I had informed them that it was being used off label and not for its intended purpose. It has also been suggested to me that you should carry out a risk assessment on the misuse and consider ways of preventing further misuse. But in reality I doubt you will be able to prevent it.