Don't agree. Mané might be quite willing to spend another year at the club (never mind our contractual rights); but be unwilling to sign a new contract. If so, the club has obvious incentives to sell him now before he enters the final year of his contract. The difference with Wanyama is opportunity cost: we're likely to forego more money by letting Mané run down his contract than we have done with Wanyama.
Ultimately these outcomes reflect the tensions embedded in our business model (notwithstanding its strengths) as much as players desire to leave: we're much more reliant on players capital appreciation as a source of investment (as opposed to owners investing new money), so our room for manoeuvre will always be more limited in cases like this.