A bundle of cash. Pretty much worthless these days. Useful as tinder.
In-game description

Cash is a material item that can be used as tinder to start fires. It's appearance is of a green Canadian $20 note - the former currency of Great Bear Island and Canada before The Collapse.

Fire Starting
Starters Cardboard matchesFirestrikerMagnifying lensWood matches
Tinder Birch barkCat tail headTinder plugCashNewsprintNewsprint rollStack of papers
Fuel BookCedar firewoodCoalFir firewoodFirelogReclaimed woodStick
Accelerant AccelerantLantern fuel