The problem is that the trade was done with two original pieces of artwork being traded.

However, what OP is getting is essentially an edited base instead.

I'm assuming had they known they'd just be getting an edited base, they would not have been so quick to have made the trade in the first place, and that's where the problem lies.

Technically the artist can do this, but it understandably feels cheap and honestly kind of skeezy to have this done to you, especially when you did not know or agree to trading for an edited base as apposed to a completely original piece.

Hopefully this explains some of understandable frustration the OP is feeling.