The only way I would do it would be to sell the D850 privately to maximise the figure recovered. For some reason, they are still fetching silly prices second-hand.
Then you could buy one more cheaply from
Panamoz if you're happy buying grey or pay full UK price for the best deal you can find elsewhere. PX deals are never worth it in my experience - it's always win/win for the dealer.
Although it's beyond me why you would sell the best DSLR around and replace it with a seriously overpriced mirrorless.