The argument for selling him at the right (High) price is to do with squad numbers.
In CL we can only have a maximum of 17 overseas players plus 4 HG players (ie 21 players - or less than 2 players per position).
At the moment for CL as CM's we have : Bissouma (need to upgrade DM), Bentancur, Sarr, Bergvall and Gray (5 players for 2 positions), so we have one player too many for CL without being short in another position.
Sarr offers a great engine and powerful running, but not too much else right now - Liverpool want him for his potential as much as what he does now.
I'd be happy to sell Sarr for say £40m/£50m to free up an overseas place in our squad for another position where we are short of players. I'd retain Bentancur for his experience (which Sarr doesn't have), whilst hoping we bring in an upgrade on Bissouma.
To add numbers to a 'Sarr less' CM squad, I'd bring in Devine who is versatile (can play DM, CM and has played both wings for E u19) and Donley to help cover injury prone Maddison and Kulusevski - both Devine and Donley count as List B and do not take up a squad place.