I'm assuming the credit note was placed on an electronic card, when you speak of it being "empty" when you went to redeem it.
If so....
Ring the store and ask for a time to speak with the store manager about this.
Once there, ask if they have a record of the transaction on the 1st July.
This should have the gift card number on it.
Check it is the same as the one you have.
If not, someone has performed a switcheroo on you.
If yes, see last line.
Ask the manager if there were any staff sales at, say, 7 pm on the 1st July.
Ask if they see a pattern emerging.
If they don't see this, check if the manager is wearing adidas.