Special Summoning the Fusion Material Monsters is an option. So you may use “De-Fusion” to return a Fusion Monster to its owner’s Fusion Deck even if you have only 1 or none of the appropriate Fusion Material Monsters in your Graveyard.
If you have only 1 of the Fusion Material Monsters in your Graveyard you cannot Special Summon it.
You may use “De-Fusion” to Special Summon “Goddess with the Third Eye” or “King of the Swamps” if it was used as a Fusion Material Monster for the Fusion Monster selected with “De-Fusion”.
After you attack with a Fusion Monster, you may activate “De-Fusion” in the End Step of the Battle Phase to Special Summon the appropriate Fusion Material Monsters and attack with them in the same Battle Phase.
If you activate “De-Fusion” and select a Fusion Monster that was Special Summoned by “Magical Scientist” or “Metamorphosis” you can return the Fusion Monster to the owner’s Fusion Deck, but cannot Special Summon any Fusion Material Monsters since none were used.
Netrep Q&As:
Q: Can “De-Fusion” be used against an opponent’s X-Y-Z Fusion Monster to send it back to the Fusion Deck without Special Summoning the Fusion Material Monsters?
A: “De-Fusion” can be used on the 4 X, Y, and Z Fusion variations. The result will be the X-Y-Z Fusion monster being returned to the Fusion Deck. No monsters will be Special Summoned from the Graveyard because the X-Y-Z Fusions are not proper Fusion Summons. It will not matter if the X-Y-Z piece exists in the Graveyard. This is similar to using “De-Fusion” on a Fusion Monster Special Summoned through the effect of “Magical Scientist” and/or “Summoner of Illusions.” Curtis Schultz, 01/17/2004