The material is quartz (engineered stone), so there's no variation in
the grade/color. It's all "best quality". It can be matched with
other pieces if someone down the road orders that same brand and
color. So that's another reason why they should give some credit!

Actually, anything man-made is subject to variations in batching
tolerances and dye lots. Even the best manufacturing process will
have these variations and will track batch numbers for best color
matching. I have seen it in Formica where I had to return a sheet as
it was a different color batch (same color number) and could not be
mixed with other sheets without a noticeable shade difference. For
this reason they may not want to accept a return if they do not have
more material from that color batch to use with it.