Here's the thing, there's a difference between making a song for someone, and giving them exclusive rights to that song.
Did they specify that they would own exclusive rights to the song when you created it for them? Did you agree to the terms?
If not, they don't own the song, you just told them they could use it (for whatever they're using it for)... Meaning you can still do whatever the hell you want with it as long as you didn't specify that they now own the exclusive rights to it.
And if you did tell them they could have exclusive rights to the song, I hope you got a damn good deal.