Voucher used

I used my two vouchers which had 500 each. I used 1 for R174.99 and the other for 314.95 meaning 1 voucher had R 325.01 and the other had R 185.05. to my surprise when I went to use the remainder of my voucher I was told the my vouchers balace was R0.00. I queried with the company that won the voucher from and I was told that the voucher had been used on the 20th, the same day i went to Sam Ntuli Pick n Pay an tried to use my voucher and chashier told me that my voucher couldn't work. pickn pay please assit with my query because I left PNP with nothing on the 20th but my voucher apparently bought stuff fore R325.01 and R185.05.