No it is not FB extortion as the buyer is not requesting anything they are not entitled to, but I will say it used to annoy me whenever a buyer said those 'rather not leave you negative' words as I see that as a veiled threat.

This particular buyer however is very nice and has been asking for advice on these boards also.

Maybe you could both sort something out together as both seem to be tryng to do the right thing, and yes this situation is real, and you have admitted this is a fault the manufacturer is aware of.

Hope you can work it out amicably.