Refusing to exchange a birthday gift.

My son got Lego for his birthday. I got a duplicated Lego set. I asked the mother who brought the gift where she got it from and she told me it was from Pick and Pick greenstone mall. I went to there and explained to the manager what happened, I then requested to exchange the Lego for another one . The manager insisted on the slip, no slip, no exchange. After explaining that it was a gift and that it is not normal practice to gif the slip together with a gift. He was adamant and refused to help me. I do my groceries shopping there every month spending thousands. One would think that taking the current economics into account that a shop like Pick and Pay will assist with this matter. I am sitting with a Lego set of R370 that my son cant use. I am very disappointed in their attitude and understanding. Is the way you treat your customers? I honestly considering taking my money spend each month else where if this is the type of service and assistance you are giving us as customers.