The club will never get a better chance at selling him.
Actually have leverage if Szczesny keep performing well. He's obviously desperate to start for Germany in 2026, but he won't if he's the #2 here. If Deco tells him that's what he is going to be then he will for sure look to move. Probably won't fetch a very high fee, but 20-30m and getting rid of his big wages counts for something and would count as pure profit in the books.
Probably not likely, but Man Utd seem desperate for an upgrade and he could be a short-term solution for them.
Even if you really like him then I'm not sure how you cannot see how this makes sense.
He's a soon-to-be 33 year old GK that's on the smaller side and has chronic knee issues. It's time to move on and sign a replacement.