Tour operator asks for undue money

Some time ago I purchased a trip from a leading tour operator in Germany (I’m resident of a close-by country).

They asked me to provide them with a credit/debit card details so they can collect €1000 fee. Their payment processing time is weird and was taking several days. Initially I provided them with a card “A” (linked to bank “account-A” in my local currency), then two days later I asked them to actually switch and use card “B” instead (linked to “account-B” in EUR currency).

Now what happened:

1. I was charged the correct €1000 fee on “account-B” ✅
2. At the same time my “account-A” was credited equivalent of €1000 ?. My bank converted this amount to my local currency and took significant part of it (currency exchange spread).

It clearly seems the company responsible for charging my card incorrectly returned me €1000 on “account-A”, while collecting €1000 from “account-B” at the same time.

Now, after a long period of time, this Tour Operator sent me physical letters stating that I owe them €1000. I responded by email explaining what happened (like above), and that I’m happy to return them €1000 minus currency-exchange fees that my bank has taken (let’s say it’s €900).

1. They didn’t understand my explanation, claimed it’s a problem of my bank which I should resolve on my own, and that I still owe them €1000.
2. They don’t answer phone calls, and respond to emails with 30-days delay (making communication very hard).
3. In the meantime they sent me yet another physical letter with charges for an “extra reminder”.

What shall I do?