Theft perhaps Dischem??

I received a gift voucher code for Dischem Baby City to the value of R800 and my husband went to redeem it at our local Dischem. The value of the first purchase was R412. When I went to go use the code again I was told that the balance has been forfeited. No one told us this! Apparently if one doesn't use the total value of the voucher at first purchase then you lose the rest. Again no one told us this!! Dischem this is appalling to say the least! So you just sit there with people's hard earned money in credit from vouchers which were not fully redeemed and see nothing wrong with that?! I demand that this be rectified from head office because clearly the store couldn't help me with anything!!