2:40 PM PT -- Sources close to Dorit say that, while she and Paul know Ryan Horne, he never fronted them a dime ... and they'll respond appropriately to his claims in court.

'Real Housewives of Beverly Hills' star Dorit Kemsley and her hubby have just been slapped with a lawsuit alleging the couple royally screwed a business partner.

Ryan Horne filed new legal docs -- obtained by TMZ -- claiming he struck a deal back in August 2017 to partner up with Dorit and her husband, Paul Kemsley, to make bathing suits. In docs, Ryan claims they each owned a third of the biz and he says he fronted them nearly $130k in merch plus $75k for other expenses.

But, Ryan says he never saw a dime after Dorit and PK sold the bathing suits for a huge profit and lined their pockets without paying him back. He's now suing to get the money he fronted Dorit and PK, plus a cut of the profits from the bathing suit sales.

We've reached out to Dorit and PK, so far no word back.