Everytime I get a butterfly bush,it dies when I want to plant it outside.
This last one,I kept all winter in a terra cotta pot.
The pot split,so I repotted it in a bigger pot,now it has wilted down.
I suspect that it just got stressed when you repotted it and the roots got disturbed. Did you set it in a protected (shady ) spot to let it recover some after repotting? If not, protect it for a while, and it should recover yet for you, and then you can introduce it back into a sunny spot.
Even seedling butterfly bushes wilt if the roots are disturbed. I usually get as much dirt around the roots to go with the plant as they shock so easily. If I can't get the dirt, I put it in a large container and cover the roots with water for a day and it bounces back. The plant is kept in shade during this process. Then I plant it where it needs to go and continue to keep the soil wet, not soggy and gradually give it more light.
Here in Alabama, the birds pick my seeds and plant for me. I find them coming up all over the place. If the plant does not perk up, you may want to prune it back slightly.