Or hookers, or Crack, or a gun (illegally).All money is fungible. You could (and probably will) ask this same question about any sum of money that Hunter receives. Maybe you (and the House investigators) should put more effort into discovering a crime, and then investigate the details.
As it is, any money that passes through Hunter's hands might have been used to buy drugs, anti-aircraft missiles, biological weapons, a prizefighter dive, a fake ID, smuggled Precolumbian antiquities or contraband elephant ivory.
Without evidence of a crime, money is just money. Everybody's got some.