First National Credit Card ripoff Sioux Falls South Dakota

I ran all 3 reports and 1 report says 1st National Credit Card Open/Never Late, the other report does not list the company and the last report says Balance due $512. I believe I have a zero with the company but I trying to buy a home I decided to end a check in the amount of $300 with the notation on the back stating if the company cashes the check this account is paid in full. I also wrote a letter stating if this is unaccptable please return check. The company cashed the check and now is asking for what they believe is the balance. The company should not have cashed the check if this was unacceptable

Yvette
LAKEWOOD, California
U.S.A.