50 Cent -- Broner Shouldn't Have Ripped Jay Z ... Take the Money!

50 Cent tells TMZ Sports ... boxing champ Adrian Broner MESSED UP by taking shots at Jay Z and Rihanna -- and says he should've just taken the $40 million offer from Roc Nation Sports instead.

Broner appeared on "TMZ Hollywood Sports" on Friday and said he felt the offer was disrespectful -- and then explained why he thinks Jay should "s**k and elephant d**k.

But 50 -- a pretty big boxing fan -- thinks Broner mishandled the situation big time. Check out the clip.