How can I trade back my Celebi from White to Diamond?

I accidentally used the Relocator thinking it would let me choose what pokemon I wanted to trade (with items as well because the Transfer doesn't let you do that) and now I have 2 Celebi's in White when I want it back in my Diamond game. How can I trade it back?How can I trade back my Celebi from White to Diamond?
You cannot trade Pokemon from Black and White to the older versions of the game such as Diamond/Pearl/Platinum/HG/SS. Once a Pokemon has been transferred or Relocated it cannot be returned back to the game it was originally from. I do not know why people don't read the message that pops up when transferring and relocating Pokemon because there's a message that pops up saying once a Pokemon has be transfer it cannot be transferred back over.How can I trade back my Celebi from White to Diamond?
You can't. You can even read the instruction booklet and see for yourself, it says you can't.