Sapp filed legal docs in Florida, requesting his child support to Angela Sanders be decreased from $2,500 per month ... because he's fallen on hard financial times since retiring from the NFL, a.k.a. he went bankrupt.
Problem is, back in 2001 -- when Sapp was a Tampa Bay Buccaneers cash-making superstar -- he and Sanders added a clause to their child support agreement that said ... neither party could "seek modification" based on changes in income.
Presumably, Sapp was trying to protect his millions.
Sanders tells TMZ she's "frustrated" by Sapp's request ... because she was fine sticking to the $2,500 a month while his pockets were getting fatter -- so it's unfair for him to lower it now.
According to the docs, Sapp is also trying to modify child-support payments for his 4 other baby mamas.
A judge has yet to rule.