Wont Honor their vouchers

I went to the Eastgate store on Saturday and had an R500 voucher and asked them to see if it was valid, the lady rang an item for R300 and said it worked. upon completing my shopping I was informed they can only honor the R300 amount as they scanned it for R300 I have forfeited the R200. the manager did not care about this issue and just told me to take it up with Discovery.