Gift voucher

I once bought items worth R600 which i later returned and was refunded in the form of a voucher. i continued to use the voucher with a purchase of R300 and i had a balance of R300 left in my voucher. Today ive sent someone to use the voucher and to my surprise they say it has no funds and are surprised that it is hand written, with a hand they can identify but cant assist as the cashier was fired. The docket presented in store shows that i have R300 left in my voucher the manager was rude and told me to contact head office if not happy with their service. Tried searching for their number all pages are not found. What kind of retail is this. urgently in need of your help