The way I see it one of three things is going on here.
1. The watch originally came with a matte dial and acts as a reminder that Rolex do not stick to any particular hard and fast rules r.e. using old parts.
2. The watch originally came with a gloss dial but was replaced with a Mk3 Matte dial at service. I know someone made the point that why would someone want to go from a gloss dial to a matte dial when the gloss dial was new at that time, well perhaps the original owner of this watch decided gloss wasn't for him/her and preferred the matte dial. Maybe they just didn't care enough to note that their dial had been replaced with a matte dial lol. Either way, it isn't out of the realms of possibilities for this to be a service replacement dial, as I understand it.
3. Steve and the OP both made a mistake by not doing their due diligence.
Personally I think its the second scenario but then again what do I know lol.
Either way, I hope you guys find an amicable solution, there's no reason why this needs to get any uglier than it already has