Akon's been holding out on his biz partner for more than 10 years, and now owes a staggering $150 million in unpaid fees ... according to Devyne Stephens, anyway.

Axar.az reports citing TMZ.

Stephens is the music exec who famously helped launch Akon's career, and his Konvict Music label - and in a new lawsuit he says he's supposed to be getting 40% of everything Akon's earned since 2006. He says their contract gives him a cut of record sales, touring rights, royalties and more.

Akon's old pal admits they had a dispute over the contract, but says they squashed it in 2015. Stephens says the problem is ... he still hasn't seen a dime.

If the math is right Akon's earned nearly $400 million in just over a decade - a pretty sweet haul, for sure. Stephens is suing to get $150 mil from that bottom line.