I Need To Cancel A Purchase.

Yesterday for Argentine Mother's Day I bought my mother-in-law a sewing machine at Garbarino's in LaFerrere, Argentina. The machine had been on display, but I bought it anyway with the understanding that it could be returned if there was a problem. The machine did not function and it was returned with the idea of exchanging for a new machine. The woman was very rude and refused to exchange the machine.  My mother-in-law told her to cancel the order and left the machine there at the store. I used my MasterCard to purchase the product and I would like to cancel the charge if I may.  My card number is 5466 1600 6597 9419, exp. date is 1/31/2012. I have never cancelled a purchase before and I hope this is the correct way.  If not, please inform me of the correct way