← The Rules
Can you exchange moldy or rotten cash?
Short answer
Yes, through the Bureau of Engraving and Printing's Mutilated Currency Division. You mail it in with a letter that estimates the value and explains how it got damaged.
Paper money buried in the ground soaks up water, grows mold and falls apart. Getting it replaced means explaining where it came from.
That's why burying it in Part 2 came with a vacuum sealer.
Sources: BEP: Mutilated Currency Redemption
US federal rules, explained for a story. Not legal advice.