That'll happen the day I have a baby by Free
Not to say that anything is wrong with Free
Just to say that ain't nothing wrong with me

from Jay-Z – Trouble Lyrics on Rap Genius

As the other two pointed out, this line means that if his girl leaves him, it will be because of something substantial that he has done (i.e. cheating with another worthy star like Halle Berry) and not for silly rumors like cheating with Free (of 106 n Park fame). And he doesn’t plan on doing anything like that because he knows he’s got the best, and recognizing this, he won’t be f*kin it up.

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