(Spoilers) During the call and the deal with Hermes we knew about the Ichor (Olymipan blood) that was used as an ink to make the Hotel, and aside from the one we will give Hermes to regain his sanity we're having I guess 6 drops to negotiate. I think that in case the negotiations go south their is noting stoping the MC from using that same ichor to grant himself a wish by adding a new section to the Original contact of the Hotel.
*If it's Asterion's freedom he wants it might be a bit complicated to do because if a contradiction happens in a contract built by ichor it might a bit dangerous and might endanger the basic functions of the Hotel (generate food ...etc) by causing a sort of paradox. But allowing Asterion to leave the Hotel and see the outside world seems doable.
*If it's an eternal life alongside Asterion, this might be easy to make. I think that if the Mc was to make a contract stating that idk he wants to become immortal to be Asterion's torturor in order to aphold the Labyrinth's mission and please the gods for eternity, the Hotel might accept this contract and since it's ichor we are talking about, immortality is attainable. Then he just has to keep on treating Asterion the same way he did up until now. (Personnaly I don't like how this contract sounds but in order to trick the Hotel it has to sort of agree with the Master's mission)