unsatisfied client

I have recently purchased an S9 PLUS in July from and MTN Store, and received a voucher of R500 with the phone and was told that even though it states on the packaging that the voucher expirers end of May 2018 i will still be able to use at any of the stores including a Samsung store , when i asked the question, until when do i have to retrieve the voucher? i was advised from the consultant that there is no expiry on the voucher and when ever i get chance i can use it. i went several times to the Samsung store in menlyn store to redeem the voucher and was keep on told that they dont have stock, finally i got a chance to go the samsung store in Mall of Africa and was told that this voucher is expired , quite dissapointing based on the fact i have being told that i shouldnt stress of the voucher being expired, also you spend so much money on a phone for quality , quality does not only come in the components used but comes in the principle, is this is the type of service one gets, totally disgusting,