Japanese credit cards have a setting similar to the minimum repayment amount, called リボ払い. You are asked to enter it when you apply for the card. The limit is 5,000, 10,000 or 20,000. Or 30,000 yuan, etc. If you set リボ払い to 30,000 yuan, then when you buy something worth 40,000 yuan, it will only deduct 30,000 yuan from you, and interest will be automatically calculated on the remaining 10,000 yuan. Entering the next month's bill, if your quota is 20W, so there is still 1W unpaid, so the available quota is 19W. Don't know if this is the case for you.