Product: Credit card or prepaid card

I am very distraught at what has occurred in the last few months. I have a credit card with Home Depot/Citibank. I've had the account for about a year. Throughout this time I have been making payments to them in a timely manner. 

What I would do is always make the payment on the due date and sometimes I would make a double payment. Citibank would always accept it as a double payment. I did it at least twice. Both times Citibank took it as a payment for the current month and for the following month no problem. That is until XX/XX/2018. In XX/XX/2018, my father had been given a XX/XX/XXXX by doctors in the US. He was XXXX of XXXX and was told there was nothing more than could be done. Because of this, he contacted doctors in XXXX and was told that there was a treatment available for him. We literally packed up and left the next day. In the meantime, I paid all my current creditors double payments as I had done in the past because I knew I was going to be gone for some time while I supported my father. Everything was fine with all my creditors except for Citibank. 

Apparently Citibank reported a 30 day late payment to the credit report for the month of XXXX. My minimum payment was only {$25.00} but I paid {$100.00}. I did not find out about this until I checked my credit report to make sure I was still on track to purchase a home that I contracted for. I saw the 30 day late payment and immediately my heart sank. On top of that, I was furious because it was a lie. I made sure I made more than the minimum payment due to the fact that I knew citibank/home depot had taken them as double payments in the past so I don't see why they didn't this time. 

Because of this, I called the customer service number on the back of the Home Depot/Citi card and actually got to speak to a manager there. He actually told me that he did see a bigger payment and that he was going to correct the 30 day late. He attempted to do so but his system would not let him. Because of this, he informed me that he would actually write a letter to Citibank explaining thew situation so they could remove the 30 day late. Citibank said no because they had to be fair to all their customers. I think that is an arbitrary decision and a bogus answer. If they wanted to be fair to me, why didn't they apply my larger payment as a double payment just as they had done so in other occasions in the past. They had done it at least on 2 other occasions. When I brought up this point, they then said that if I had made the payment one day later, their system would have recognized it as a double payment. That to me just sounds like more hogwash. So because I paid it one day too soon their system took it as one large payment instead of double payments like it had done so in the past? 

What's worse is that this late payment now disqualifies me for the home I had contracted just a few months prior. I told them this and they still won't remove this 30 day late payment that I never paid late. I paid them in advance. I feel it's wrong that I am being punished arbitrarily. I made preparations for leaving out of town and am still being punished for it. My father is dead now, my credit is damaged due to no fault of my own, and now I won't be able to buy my house when its finished in less than a month. All because CITI decided not to take my payment as a double payment as it had done so multiple times earlier in the year. 

Please help me as I don't know what to do. Even the manager at customer service was trying to help me reverse the 30 day late payment so I know that I am not in the wrong here.