It's before tax I'm pretty sure. You can see the tabel I posted just above, the salary of all the extended players are there.
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I actually like the extension.
It's important to keep on to your important players and he grew into one of those while here. He's still young and got room to develop even more, sometimes we just tend to forget that as he's been here since 2015.
15-17 mio sounds like much for him, but with the form he has showed in the last 1-2 years and the importance he gained it's okay I'd say. Not to forget if he left we had to find the right replacement which isn't always easy.
And this is extension is also very much because of his winning goal in the 2020 CL final lol.