It all depends on the situation. Sometimes the club owes a player (imo after a few season of loyal service) but sometimes not (yet). In the case of Sanchez I don't feel Ajax already owes him. In all honesty, Ajax also doesn't need the money hence why they can ask for such a high fee.
So in the end the club only will lose a very very good player who is almost impossible to replace. Ajax can't just buy anyone obviously as the Eredivisie isn't as attractive as the EPL and Ajax neither can offer wages that are going around there.
It is also a little bit frustrated to see a player leave that soon. But as said, if he wants to go I'm sure the club will let him go.
Physically I already rate him higher currently. But he isn't perfect by any means yet. He still has quite a few moments where can act so energetic in his play that it is almost reckless and he starts making errors. He did that also a few times against Nice in the CL play-off we God knows how lost on away goals.
Not at all any dislike from me towards Tottenham, I actually like how some players as Toby, Jan and Chris developed there. I feel Tottenham since a few years really took a great approach on their transfer policy with the focus on developing young talented players. It's very similar to Ajax, great to see.
It's Eredivisie. But thanks ;-), we will need it. Lost our opening match yesterday.