Probably not because a 21" low profile tyre available in a compatible size for the 850,would be difficult to find,very expensive and may not even be commercially available!
The 850 on 20'" already needs to use a 30 profile tyre,so in 21"..this would need to be a 25 profile.The closest commercially tyre available I could find is 295/25/21,which is only +2% off the 265/30/20 an 850 would have in 20"!
The offset shouldnt be too bad as in 20" ,the 850 ,has a ET24 .
Best thing would be to get the 850 guy fit your wheels on and then he could decide and you sell it as a "buy as seen" )