Business ProfileforMuji USA Ltd

I bought a shirt with an (unaltered) price tag which priced the shirt at $29.90 When I got home I saw I was charged $39.90 for the shirt. The next day I returned to the shop to receive a refund for the difference, which was in error. However, the clerk insisted that the shirt should have been priced at $39.90 and that they would not issue a credit for the $10 difference. She told me that they had placed stickers on all of the shirts updating the price to $39.90. Although my shirt was clearly not among them, I went downstairs to check the rack. There were 9 of the shirts left on the rack. 5 shirts, like mine, all had unaltered tags priced at $29.90 (same color, model, etc). The remaining four did indeed have stickers over the price tags reflecting the higher price. I brought the 5 shirts upstairs to the same clerk, but she remained firm that they would not issue a credit. I dont know the legality around whether stores can charge a different price at checkout than what the tags reflect, but I do feel that this was not right.