Double Penalty Charges Despite payment

I booked my flight in Nov 2012 for Kenya from Washington DC. A few days later my niece changed the return date to 6 days later.This included a penalty of $300 which I was happy to pay.

The Flight itself was okay we left DC on time and 11 hours later arrived in ADD and after a 2 hour lay over we left for Nairobi.

My return trip is when my nightmare begun !! January 15,2013 I arrive at Jomo Kenyatta airport in Nairobi,Kenya and a gentleman working for Ethiopian Airlines says I can't board my flight unless I pay a penalty of $300 because I changed my flight dates.

I explained to him that a payment of $300 had already been made and I pulled out my Itinerary and proof of payment which he disregarded.
This man was rude and had the worst customer service I have seen in a long time.My wife tried to reason with him and he said nothing was showing on his computer and if I wanted to board that flight I had to pay the money.
I saw no reason to argue further I paid another $300 in cash and got a receipt.I arrived in Washington and had my niece call their offices we were put through a call center in India and we explained what had happened.They asked me to scan the receipt and wait to hear from them.

A month later Feb 2013,I got an email from one of their staff here in DC saying they were looking into this and they would get back to me.
In March 2013 I emailed and said I hadn't heard anything.I never got a response.A week later my niece who booked the flight called their call center in India and we got a direct number of somebody I would be dealing with here in DC.

We called the number and we were told the person dealing with the case was on holiday and she would get back to us.That was two weeks ago!!

Six months later and Ethiopian Airlines have not refunded my money.They should be ashamed of themselves.
one thing is for sure I know I will never fly with them again.I will prefer to pay more money and fly with a different airline.