$50 off of $150 spent at either Diesel regular stores or Diesel outlet stores when you check out with American Express card in the US (not sure if it works in other countries) before 12/24/2014. You have to have a Twitter account, sync your American Express card to your Twitter account, and tweet #AmexDiesel.
has all the info on how to sync your Amex card with Twitter and other similar offers like this one.
has the info pertaining to this Diesel offer.
Amex offered this same cashback deal back in March or April. I bought something then and got $50 in credit on my Amex bill. So it seems Amex offers this every now and then.