AT&T Broadband DSL price reduction not applied to bill as promised Internet

I called AT&T to tell them that I got a better deal on DSL Service through our cable company.  The first AT&T representative told me they had nothing to beat their cost.  I asked to speak to the customer retention specialist since I've been a AT&T customer since 1988.

The customer retention specialist offered to give me a 50% discount on my DSL service for 6 months.  I agreed to this.  When I received the next month's bill, I noticed my bill had gone up not down.  There was no reduction applied to my DSL service.

I will call them in the morning and ask about this, but I wonder if AT&T thought I wouldn't notice this.  The cable company offers faster broadband plus telephone service cheaper than AT&T.  And I am almost certain if I switched to cable service, AT&T will offer me money to switch back next year.