In a vacuum, it's totally fine. Acerbi has played well and deserves to stay. In an ideal world he would be redeemed for a nominal fee and stay at the club to provide experience as a reliable backup option.
However, when you look at the wider context the whole thing is fucked up.
At this point, Acerbi is a starter ahead of that absolute wimp de Vrij. To be real though, Acerbi should be a backup or rotation at best for any serious top side. Absurdly, despite being benched by Acerbi, who himself should be a bench player, we're still trying to renew de Vrij.
When you consider Acerbi's transfer fee, plus his and de Vrij's wages, it's a significant amount of money being spent on two defenders, neither of whom should be a starter - all the while losing an actual starter and our best defender for free.
It's just ridiculously bad business.