Never accept stained bank notes as they have likely been discoloured from ink dye used in security devices such as ATMs.
ATMs hold cash in special containers that protect cash with dye-stain technology that is activated when someone tries to break open the container. Once activated, the cash is stained with a green dye, thus defacing the notes, rendering them unusable as currency.
The stained notes are recognised as having no monetary value once they are stained.
People who have these notes make themselves suspects of a criminal investigation that will seek to determine if they were involved in the stealing and unauthorised access of these ATM containers.