The way I see it, there are four ways this could go:
1. We decide that Kulu's worth buying this summer, and fork out €30M for him.
2. We hold fire, he plays half the 2022/23 season AND we qualify for the CL at the end of it, so we have to pay €35M for him.
3. We hold fire, either of the two conditions doesn't happen, and we decide to buy him for €30M.
4. We hold fire, either of the two conditions doesn't happen, and we decide to let him go back to Juve/wherever.
Honestly, I don't see #4 happening unless Kulu picks up a serious injury or somehow becomes rubbish.
But what I'm worried about is some situation #5, whereby we don't buy him this summer and we lose him to some other club. Based on his performances so far, I'd say sign him up ASAP. We've spent far more on much worse players. If the contract is watertight enough that there's no danger of losing him then, sure, wait a year so we save some wages. But it would have to be some serious legal stuff to prevent him from going elsewhere.