SUPAQUICK *****S TIRES

I recently bought a full set of tires from supa quick Stanger, with requests of having my old ones returned to me, the guy at the cashier asked if i could pick them up in the morning since it was closing time, when i got there the next morning they said that they had misplaced one of my tires and there is nothing they could do about it , I have been asking them for an explanation and how they plan on crediting me since those tires where still usable, they only keep saying they got nothing .