Returning an engagement ring

My partner bought an engagement ring for me as a gift. It couldn't fit as it was small. Tried returning it to the store the following week for exchange and I was told that I cannot be helped without the customer slip copy meanwhile they have their own copy at the store. Asked them why can't they use their copy and they just told me that they can't 'cos they're not allowed to. It's been a week now and I still have the ring with me.