You will retrieve some credit, but if you can't prove what you paid, you will receive store credit of the current sales price.
You'll get the lowest price they sold it for.
If you paid with a debit or credit card and take it back to the store you purchased it from they can look the purchase up with the card you paid with. Otherwise you will receive credit for the item at the current sale price.
Without a receipt, I believe they'd only offer you whatever the current price is, even if it's on clearance or sale.
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