The difference between hustling a non-exclusive and exclusive beat is relative.
It depends on whether the other party is willing to invest in a lawyer to stop you.
Probability is that they're not, and if they have that kind of money, they will try to contact you and do a legit deal aka buyout-- with an actual lawyer.
Is someone going to really sue you cause you sold a beat? (if they're in your neighborhood, they might kick your ass, but that's a different story.)
The music business is pretty amoral. You have to be ruthless to survive (but pretend you're actually not ruthless and a "good" guy).
Time for the disclaimer, just in case:
(this post does not constitute legal advice, get a lawyer if you want advice.)