Damon Dash -- Jay-Z's Ex-Biz Partner ... ROC-A-TAX LIEN

Damon Dash probably wishes he still had Jay-Z on speed dial ... because the cesspool of debt he's wading in just got worse ... he's been slapped with a $2.8 MILLION TAX BILL.

Dash -- who co-founded Roc-A-Fella Records and Rocawear with Jay -- has been open about the fact that he's drowning in debt ... despite the fact that at one time he was half-king of a multi-million-dollar business.

Now, his financial problems have worsened. Uncle Sam filed a federal tax lien against Dash, claiming he needs to cough up some un-paid tax cash for the years 2005 and 2011. The breakdown:

2005: $2,614,918.10
2011: $187,544.20

Grand total: $2,802,262.30 ... i.e. a whole lotta cheddar.

The 2005 bill is clearly larger ... which makes sense considering Hov and Dash parted ways back in 2004 -- meaning in 2005, he was probably still raking in some major-taxable bucks. 2011 ... not so much.

FYI -- Dash has said he owes millions in back taxes to the State of NY and NJ and claims he's lost multiple properties to foreclosure.