Uncle Sam
has a bone to pick with Kevin Federline -- and it's not for
releasing "PopoZao" -- it's because the IRS says K-Fed still owes
back taxes ... to the tune of $57,615.39.
A federal
tax lien was filed against Britney Spears' baby daddy... claiming K-Fed has outstanding income tax bills for the years 2009
($30,517.16) and 2010 ($27,098.23).

