Is there a federal statute on how to apply payments?

I had a medical insurance arrearage judgment against me in 1998
relating to insurance I was never ordered to pay in my 1988 court order
-- yes, a retroactive modification.

Last year, because I'm headbutting Virginia's system, a JDR Court Judge
ordered that DCSE could have interest on that $6168 back to 1998 when
the retroactive modification was made to my 1988 court order.

Although my current support obligation ended in May 2002, DCSE is
paying off interest on the child support arrearage before the apply any
payment to the medical insurance arrearage, which is continuing to
accrue interest.

Does the federal government have any laws relating to how payments must
be applied to different forms of child support, i.e. interest on child
support, medical insurance arrearages, interest on those arrearages?

I've emailed the District Manager of the Manassas District DCSE Office
and have never received a reply to the same question.