exchange refused after we agreed on exchange

Two months back I bought a cellphone at the store and the sales person insisted that I buy a screen guard at their shop which he claimed were good quality and cost me R150.the consultant had difficulty purring it on my phone and two days later I had to return it because it was peeling off.on the return of the product I had to pay an extra R50 to get one that was \top of the range"I got a new one and immediately it was peeling off so we agreed that once new stock came in I would get a new 1 "