Gives a voucher and then cancel it without notice??

So I upgraded to the S9, got the watch included in my package. Also the R500 accessory voucher. The voucher only expires 31 May. So I went to an MTN shop to claim it and then they tell me MTN gave 1 month chance to redeem all vouchers and then cancelled them. How? When was I informed?? Would love to know how you can give something and then just take it back without notice??