"You're going to vanish? Well, if this was a temporary summoning, you wouldn't be able to bring any gold back with you...No, as I said before, this is a Planar Binding effect. That means you won't be able to leave until you have completed the agreed-upon bargain. It also means that if you die, you will be destroyed forever. So tell me, do you really want to press your luck, little beast? If we command you, you must perform the task. For 50 gold, the task we demand is to open the passageway and then sit down there," she points to a spot on the ground, "And explain everything you know about this situation--including, but not limited to, why you know how to open the path, why the altar had a ritual that summoned you, how to make sure the door will not close on us on the way out and the secret of how to open it for us in case we need to do it, what is really sealed within, what happened to this place to cause it to be abandoned and full of foul and most unnatural creatures, and all the rest. You must know the answers else you would not know how to open the way. The magics will keep you there until you complete the task. That also means that once you accept the money, you will be magically bound to complete the task in its entirity. We task you with that task. And no weaseling out of telling us everything--you must answer all those questions as well as state everything you know about this place and the situation here."
(OOC: If it acquiesces, she's okay to have them pay the gold. The idea is to listen to its information and get it on the ground. Then we can probably kill it while it isn't done yet. Interestingly, though Aletheia thinks it's a Planar Binding, it's probably a Planar Ally effect, which is written assuming the summoned critter is of an alignment friendly to yours (since its Cleric only and Good Clerics are banned from using it to summon evil outsiders), so they don't have any sort of clause for enmity)