It's really not a slamdunk decision to pay Sanchez 4m€ to get out than keep paying him his salary if he keeps performing like he has done. If he just manages to stay injury-free, his salary is fine. Since he's an awesome player. But it's a big if.
To reiterate I think Sanchez gross salary is around 11m€ and the release clause is probably 4m€ net which turns to 6m€ gross (Growth Decree). So the gross difference would be 5m€. Who can we sign with 5m€ gross to then replace Sanchez? Not fucking much.

That's like 2.5m€ if from Italy and less than 3.5m€ if from abroad. And remember, only free agents. If signing fees are added on top, we can sign even lesser talent with 5m€ annual cost.