Half Price Books North Austin Location, Parmer Crossing Would not honor their "same as cash" voucher, for sold DVD's/Bluerays Austin Texas

Half Price Books, (North Austin) location Parmer Lane, would not honor a cash receipt voucher, "credit" for several movies I had sold the location at an earlier visit to the store, I had sold several high end Criterion Blue Rays, approx. 6, and several good regular DVD movies they gave me nothing for, I thought I really would like to pick something out, but could not make selections at that time, I came back about a month later. Made 3 selections, they scanned the barcode, and said it had already been redeemed.  I talked with the manager and they said I had either made selections when I initially came in or got the cash.  This was not the case, 

I was assured to not lose the same as cash voucher, because in effect thats what it was, I then came to the conclusion, after a period of time, they just liquidate the credit, thereby nullifying the supposed same as cash voucher.  

In this day in age of Brick and Mortar establishments under threat, is this the ethical level of Half Price Books?  Is this a reputation they want to double down on?  The 23 dollar or so, voucher, is not so much the point, More to the point is Half Price Books policy and false projection of latitude with a customer who more often frequents, the high end stores and enjoy the "experience" of shopping in person for items, and adding to my movie collection,  Their Policy or failure in accounting, should be reviewd internally.