You give me a gift and than take it back! Dissapointing

I was excited to go redeem my birthday cake voucher on my app wich I got for my birthday on the 12th October valid for 30 days..When I go redeem the voucher on 1st November, I'm told I cant redeem it because rules have changed. So they give me a gift and than decide no we are taking it back! I would have understood if the voucher was issued after rules have changed. I walked out and went to buy cake at the nearest coffee shop, doubt Mugg and bean will ever get my money again..Im very dissapointed