Poor customer service

Went to Spar (ghandi square branch ) and The cashier (joice) on till no 4 refused to take my bank note and said it has oil.which I had just taken it out of the atm next door the spar.how can she decline a note from the atm..went to woolworths with that note they never had a problem so why did she have a problem with my bank note ?or she had a problem with me