Credit - Vodacom voucher

So I ordered headsets and a jack pin and paid with a R500 vodacom voucher and cash. When I returned the headsets the credit was of the amount paid in cash and not value of the purchase. The value was R1220 but only got R797 and the R500 diffrence of my voucher was not credited back to me. What informs the decision to keep that R500? That voucher was for my benefit not Takealot's