Usman Vs. Masvidal NOT for BMF Belt

Sorry, Kamaru Usman ... your UFC 251 fight against Jorge Masvidal will NOT be for the BMF title -- so says Dana White.

Of course, Jorge won the BMF title in his UFC 244 (Nov. 2019) victory over Nate Diaz -- with UFC spending a TON of cash (around $50k) to get a special championship belt made.

Jorge hasn't fought since ... and now that he's finally getting back in the octagon, everyone wants to know if he's putting that belt on the line!

In fact, Usman -- the reigning UFC welterweight champ -- recently told us how BADLY he wants that belt.

But, bad news for Kamaru ... Dana says it ain't happening.

"It's not on the line," White tells TMZ Sports ... "Usman wants it to be on the line so bad, he will…