Kevin, I assume the slab in question is inside the home (it's a "slab home")? I'd want to know all I could about this crack, and exactly how & when it was repaired, and by whom.
From what you write, the agent has made a pretty "general statement" that doesn't necessarily mean that the repair to THIS home was done properly; and what constitutes "properly" will vary with what the initial problem was.
I also notice that he/she has compared a home with a repaired crack to a home with an unrepaired crack, not a home with NO crack; big difference in meaning there.
The crack might well never be a problem again, but there's always that chance. If it's merely a hairline fracture with no side to side displacement, there's no real worries here.
Kevin, let the agent know that before proceeding you'd like to have some additional background on this, and get an independent home inspection done when you can provide some details of the work to the inspector. Do not use an inspection service provided by real estate agents; get one who'll be working for YOU. Once you have an independent opinion of the home, you'll be able to decide whether to adjust your offer, let it stand, or withdraw. Good Luck, and post again with further detail if you have more concerns.