So what happens if she's summoned and the summoner doesn't agree to her terms or want her? Does she just go right back into Sensory Deprivation Hell again? I mean, that seems damn cruel. Imagine her finally getting out after 300 years and not having any gold or magic to offer because of the limitations and thus getting sent right back.
I mean, there's always the hentai doujin route of "pay with your body" which carries its own ethical concerns, albeit with a 300 year long dry spell she probably wouldn't be too upset at such a proposal.