Thank you Nicolas & Tcook.
Yes the discount was definitely valid on that particular day. The "lovely" manager told me that he had some other tables with the discount that particular night.

Unfortunately we made the booking on an Iphone at the last minute and didn't print anything so there is no way to prove anything

I do understand that mistakes may happen but I was mainly disappointed with the way the manager deal with the situation.

I sometimes wonder how "real" are these discounts, I have the feeling that some restaurants increased the price to afford the 30% or 40% discount and if some clients pay the full (regular / already increased) price that's a "plus" for them.