Why is this happening?

[kostis_from_kazia]

I just sold a player for 20.5m. However, I didn't receive a single penny. Why did that happen?

[IbrahimAliMaher]

Your chairman is a crook 

Assume your % transfer revenue received is >0? Did your wage budget change at all? Any other incomings/outgoings around the same time?

[kostis_from_kazia]

My transfer revenue is 100%. And he was the only player leaving at that moment.

[michaeltmurrayuk]

Have you checked to make sure he has actually left instead of it being a future transfer?

If you check the finance screen you should be able to see if you actually got the money to your balance and if the board have just decided not to add it to your transfer budget, the finance screen on the right will also tell you if the board have limited your max revenue from transfers, also check to make sure your transfer budget isn't set all the way to wages or scouting budgets as the extra money may have be allocated to them and you'd need to move the sliders over to transfer some back to your transfer budget.