You don't need a law degree to know the laws of your industry...
I'm a contractor by day. If I go to a job, and the carpenter says, "Hey.. can I use 2x4's as the roof rafters? 2x6 and 2x8 are just so heavy, and more expensive. It's fine, right?" My answer is NO, that's not code. Now, I'm not an inspector, and I'm not a lawyer, but I KNOW that's against legal building code.
Same applies here. If you're gonna rap on a beat you don't have permission to rap on, it's just illegal, straight up. You don't need a law degree to know that. In fact, as musicians, we SHOULD be aware of the laws that apply to us, regardless. It's part of your craft, whether you're into it, or you like it or not. If your argument is "Well, nobody is gonna know.. just do it," then by all means, give this guy some free beats of yours. You're a hobby producer? Hook up with this hobby rapper. No harm, no foul.
Again, going back to the contractor analogy, just because your buddy will build you that deck for free coz he's a nice guy doesn't mean I will. I'm trying to make this my livelihood, like many have before me.
If a producer has a beat that says, "Free for use" or "Free lease", that's fair game. But if you're buying a beat through iTunes for $1.00, it does NOT grant rights to re-use the track without consent. Same goes for free preview downloads on SoundClick and other sites. You have to do business with the producer directly and correctly if you want to alter a piece of their music by rapping over it - even if you don't want to sell your finished product.
There are enough producers out there who just want to see their beats rapped on, for their own promotional use. Hook up with some new beatmakers who could benefit from free promo. That way, you won't have to infringe on someone else's work.
To the OP, I would check up on some of the language and circumstances that come with buying, selling, and releasing music online. Exclusive vs Non-Exclusive Rights. Royalties. Copyright. Creative Commons. How producers make money. How rappers make money. Etc. Again, even if you don't want to make money rapping, you can certainly lose money by making certain uneducated decisions regarding other people's music.