The Hero's Path

we can agree to pay 30,000 not immediately, but "upon completion".

There is nothing more to pay here; when you complete the quest with the wizard, when choosing to pay upon completion, he says that builders came to him and extorted money, so the reward is less (by the same 30,000).

My sphinx only complains about his difficult fate and sends me away.

What is the status of the "Wizard's Option" quest? Perhaps you didn't receive it from the mayor in Archild?