But they didn’t take the money out of Revlon’s account. They just paid it from their own funds.
That's irrelevant here. "Money" is just bits in some DB. The point is, BoA sent money to the CC company, like they always do. How is the CC company to know where the money originated from?
The same way you'd know your employer overpayed if your monthly deposit somehow skyrocketed by 10 times. No you didn't get a 10x raise, there was a mistake somewhere down the line.
Also, they know simply because BoA told them immediately. I can't believe people advocating what is clearly stealing.
But Citibank was not "someone else". Citibank was Revlon's banker.
Imagine if you used BankOfAmerica to pay off your credit card bills monthly. Now imagine that you owe your CC a large amount, but are paying it off month-by-month (using BoA, as always). One month, BoA takes all of the owed money out of your account and sends it to you CC company, thereby paying off your CC debt. How is the CC to know whether you meant it or not?