Cash did not seem to do anything in the match. It seems its sole purpose was to be used in one of the Welcome to Pandora mission objectives. However, it is technically listed in the files as a type of ammo. This makes it the first type of ammo that doesn't have a weapon. Uncommon rarity ammo types are usually infinite and cannot be seen in a player's inventory.
Cash was removed from Borderlands Chests in Patch 10.30 along with Pandora, though it can still be found in Creative using glitches.