Mislead by store and customer service

I took the lg g3 mobile in att next plan in store in Feb and was told there is a 100 $ credit for the phone and it will be credited in 4 months.. Now I am calling customer service and store for 4 months and every month they say it will be credited and finally this month they said there is no such offer.
I feel totally mislead... Now I can't prove there existed an offer. And they have been just playing with me all this time.