Citibank Credit Card Complaint

Citibank Credit card department,

Payoff process California

I purchased a hot water heater from Home Depot. At the time of purchase, it was a special promotional offer, no interest if the balance on the item was paid off within a specific time. I made a payment on my regular balance, and wrote a separate check to be applied to the promotional balance only. They did not apply the second check to any of the promotional balance. I have called them twice, requesting that they do so, and they have told me yes, they would change it, but they have not. On the most recent call I made, I was told the change would show up on my next statement, which would put it past the promotional offer payoff limit, and they would then charge me back interest. This is unacceptable, and should be illegal.