That doesnt add up. Even doubling his salary from 200 to 400k would cost us 10M, which is far less than hes worth if we would sell him. Financially it would be idiotic
£200k to £400k would be £10m per year; plus a signing bonus (another £10m? More?).
1) Let's say next season costs us existing ~£200k/week; £10m. No contract. No sale. £10m, for a year of Salah, at his peak, but then we have to replace him (with a year's advance notice).
2) Let's say a new contract, as above, signed now - £90m (or £100m), over four years. No sale. Four years of Salah; at £22.5m/year (or £25m), spread. Then he's 34 and we may want to replace him, whether we have to or not. We don't know for certain how long his peak will last. Have we really protected his sale value at all, when considering doubling his wages, if we later wanted to sell? Note, 2022/23 Salah now costs us £22.5m (or £25m) instead of £10m.
3) Let's say we sell him this summer (ignoring his comment that he's not leaving). £50m? £70m? More? Less? No Salah next year. We have to replace him immediately.
I think that's rather less clear cut than we usually think about a player running their contract down. We might have a player lined up, or even in the system we're happy to be taking that role in 12 months. The difference in cost to the club between running the contract down vs total cost of a new contract is ~£80m, not £10m. If you think he's still going to be world class at 34, that's not a bad deal - unless you have a better/cheaper alternative. But at this point, the chances are increasingly that his best season is the next one, not the one in three or four years. But arguably the best deal, with the most certainty, is having a 30 year old Salah for one season at a cost of £10m. That's ridiculously good value. Sure, we could sell (if he agrees) for £50m or so, but that would only make sense if we could replace him (at similar or lower cost) without significantly impacting performance.
This stuff is complicated. I don't mind what we choose to do as a club. If I had to choose one, I'd probably sort a deal for Mane (and it would be cheaper). The club will certainly have more nuanced and complex value and opportunity cost modelling than this, but it's not just the 'loss' of not selling him now vs leaving on a free.