Have a slice on us - or not

My birthday was in December and I received an SMS on 20 December where Mugg and Bean invites me to come "have a slice of cake us". There are many other restaurants I could have gone to, but because of this friendly invite, I decided yesterday (5 January 2020) to go to Mugg and Bean in Tygervalley. When the waiter showed me the menu, I first checked with him if I can claim my promised slice of cake, and Emmanuel agreed that it is all above board and I ordered our main meals planning to have the cake for dessert. Because of their generous portions, I decided to have the cake as a take away, and asked for the bill. When it was time to pay, their wonderful "Generosity App" couldn't process the discount for the cake. After a few attempts my waiter called the manager who also couldn't resolve the problem. The waiter said there is nothing he can do, even the manager was there and she couldn't assist. I asked him if he couldn't just make out another receipt, and exclude the cake, which he did, but then he took the cut cake back probably because I did not pay for it! I then paid for the food and drinks and left without the "slice of cake on us"! My frustration is that I choose the Mugg and Bean because of the free slice of can which I now feel was false advertising, and I wonder how many people didn't fall in that same trap. And what were they going to do with the takeaway in the bag? If they were going to throw it away, why couldn't they just give it to me, or were they going to keep it and hope another client orders it... Very disappointed!