The sP you need to have to take a quest from Saggitarius or Sarah is erroneously being applied if you need to turn in a quest also.

So, if you had the right amount of sP for Saggitarius, took a quest, then bought the needed items, he won't let you turn in if buying the items put you below his threshold. Sarah has the same issue in reverse.

It's been like that for years unfortunately. You need to make sure you'll have an appropriate amount of sP to turn the quest in too. Or try making or spending as much as needed to be able to finish the quest.

Not sure what staff had said about it working this way. Personally I think it's something that should be fixed, unless there is good reason why it hasn't been already. Maybe there's some way this can be abused?

This is not an error, it is intentional.

It is not something to fix. It is supposed to be this way. If you go below or above, then you need to go back above or below to finish it. Locking as this is not a bug.