This article is from the Final-Fantasy and other Square Soft Games FAQ, by email@example.com (Nick Zitzmann) with numerous contributions by others.
Ryu-O has the other orb required to get the Sphere for Floor 5, but you
must solve his riddle to get the Sphere.
He'll give you a list of items. Write this list down somewhere, and go
look up the price for each item at the store in the town near the Tower.
Add the prices up, and buy an item whose price matches the number you
got when you added the numbers together. Take the fresh item (don't use
it!) to Ryu-O and he'll give you the Orb.
If it still doesn't work, you may have to put the item in possession of
your party's leader in order for it to work.