Zudio — Credit note raised for returned cloths

I made a purchase returns for Rs. 650/- about 6 monts back in zudio - bandlaguda branch, nagole, Hyd . The employee has given a credit note and said i can purchase anything on the given credit note. But, recently when i went to zudio for purchase they said my credit note is expired and cannot purchase or avail anything. I don't understand why there is an expiry date for credit note given to me. It is the Zudio responsibility to give me the money without any hastle. This is a Scam which Zudio is doing or employees at Zudio. May be my money is less but i insist a Verification need to be done by organization to avoid these kind issues/frauds.