How would one handle this scenario if it happened?  (This has not happened.)

I’ve accepted a job that hasn’t started yet and I have no clue when it will start.  Say, I find another job in the meantime that pays more and they say “Sista…come work for us.”  How does one handle that scenario?  Do I say, “Can’t…I’ve accepted a lesser paying job?”  Or do I tell the other “Sorry, I found something else?”