@normchevy,
You truly can not go after the over payment and they will make you to look and to feel like dirt if you try.
It is also possible that you might have miscalculated since the Child Support is usually based on a monthly basis while there are often 5 weeks in some months, so if you try calculating the c/s per pay period or per week (as 4 weeks per month) then it will often appear inaccurate when it is really just the law garnishing to the very last dime.
Sorry for being negative, but the Child Support is a negative business.