I'd take the discount. You will then have your stock wheels AND $3k savings. You can buy the black wheels for $2.1k plus shipping, swap your tires over and sell your stock wheels... You'll come out ahead.
If you want to keep pushing with the dealer, I know for a fact they fit because I did what you're doing. I had them swap for black GTS wheels on delivery. Absolutely no issues. I had to pay tho ($1.5k), which is BS, but it is what it is....
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Current: MB 21 X5 M Comp | '20 Z4 M40i
Past: 09 M3 Coupe DCT (sold) | 11 1M Coupe MT (sold) | 15 i3 REX (returned) | 15 X3 35xi (sold) | 16 M235xi Vert (returned) | 18 M3 Comp Ultimate DCT (sold) | MW 20 X5 M Comp (totalled)
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