This isn't really a bug, but I thought it'd be relevant to say because, at least in my point of view, this shouldn't be happening. I was completing Sarah's quests when my sp went slightly over the maximum allowed for her quests and I switched over to Saggi. However, after buying the items he asked for, my sp was low again and when I clicked to finish the quest I got an error message for being too poor. I had to complete one Sarah's quest for my sp to be above the minimum so I could complete the Saggi's quest that was already running. It wasn't too much of a problem, but if I had completed all other quests before Saggi I'd have to play other games and it'd take a while to get the amount I needed. I wonder if there's a way to stop the error message from appearing if a quest has already started.

I think this is intentional. If you want to do Saggi without trouble then just keep 20% more than his minimum.