bad service

I bought a Defy 20 liter microwave on Monday the 7th of April cash. When i got home i realized that the microwave is small and I have asked my husband to return it today since its his day off and buy a bigger one/correct size. My husband is at the store right now and they are refusing to assist him in changing the microwave to buying a bigger one. He was given this contact numbers to call ***/2101. I called the first number and its even a wrong number. How unprofessional can they be??? i dont want a refund, i just want a bigger micro wave and will add on the extra money, why are they refusing to assist me??? Spoke to Refilwe from the store who is supper rude and she is still refusing to assist my husband, and They forcefully wanted to lock my husband inside store and gave him wrong head office numbers.