From what I've read it's supposed to be an optional fee if Spurs fail to qualify for CL that becomes mandatory if they do.
FWIW, despite me winning the prem & qualifying for the CL while having spare funds I also never had the option to pay the fee and he went back to Juventus with a value over 3 times the original fee. There wasn't even a button or way to bid for him either like there usually is(?).
I could for example buy Dean Henderson while he was still on loan at Forest, but there was no way to bid for Kulusevski.