The 5th spell Wish has two main uses:
- Duplicate the effect of a spell of level 8 or lower, or
- Create another (perhaps stronger) effect and suffer a "magic backlash".
If you asked the wizard to use its level 9 hex location to reproduce the effects of a spell such as Major restorationSo, basically, you get a free effect from it, without it (or anyone else) needing to provide the 100gp diamond hardware component; you could owe him an equivalent amount of gp for convenience, since 100gp is [probably] more practical to acquire than a 100gp physical diamond, plus possibly a fee for using its only level 9 spell location for the day, which it might otherwise have used for another use.
Alternatively, he used it to reproduce spells like Snacks, Dispel the magic, Remove the curse, The empty mind, or Cure, none of which contains material consumed, expensive or otherwise. You could therefore owe him a small pittance for his time (in addition to the aforementioned opportunity cost associated with the use of a level 9 hex location).
However. The wizard may have used the desire to reproduce an effect that can not be described by a level 8 (or lower) spell, if the effect placed on you was particularly difficult to remove or dispel. In this case, the wizard suffered 2d4 days of retaliation (average of 5 days) and presented a 33% chance never be able to launch a new Wish. This is a huge burden and a risk that the assistant has taken to cure your illness.
At this point, it depends on what your DM decides, but what your DM will take into account is the generosity of this wizard (is it a good Samaritan who felt that you deserve the help you received and paid no attention to the consequences on him??) and how ready they are to suffer the potential consequences of never being able to launch Wish never. However, in most cases your debt to the wizard would be quite extraordinary.