Russell Simmons' Biz Partner -- You're Screwing Me Out of MILLIONS

Russell Simmons' and Reverend Run's business partner has declared war on the hip hop moguls -- claiming they've hijacked MILLIONS in profits from their kids shoe company ... and now, he's gearing up to sue.

Lawyers for Rasheed Young -- President of Run Athletics -- sent Russell and Rev. Run a letter back in November, accusing the brothers of secretly funneling millions in revenue from their Pastry brand of footwear ... into a company that Rasheed isn't a part of.

Rasheed claims Pastry has raked in tens of millions in sales -- and so far, he hasn't seen a penny of it.

Sources close to Rasheed tell TMZ, Russell and Rev. Run have been stonewalling him ever since he sent the letter -- refusing to respond to Rasheed's demands -- so now he's cranking up the legal heat.

Rasheed's demanding the Simmons brothers fix the screwed-up business structure ASAP ... and pay him his share of the shoe profits. If they don't, Rasheed says he'll sue.