Bad practice AT&T

Recently I change my AT&T cel phone under the installment plan promo at the AT&T store. At the store the rep told me that if I sign for the 12 months installment plan at the end of the plan the unit was mine. In addition, the extended price was almost the half of buying the unit without any plan . In the receipt they charge me the $724.99 , when I ask the rep he said that was only for receipt purpose but my price was 12 x 35.00 aprox . or $420.00.
Now, AT&T tells me that is not like that . At the end of the 12 months , if I decide to keep the phone I need to pay the difference to the full price - $375.00. 
I don't want you the another victim of AT&T misguiding marketing - before making anything with this company , ask for a written document with all the details , take it back home and study it . Avoid being another victim!