Legal analysis: it will not turn into a crime, because the perpetrator is really paying back the money, and there is no "for the purpose of illegal possession". Those who maliciously overdraw for the purpose of illegal possession and refuse to return it after collection can be convicted of credit card fraud; Because the amount of 300,000 yuan is extremely huge, he can be sentenced to fixed-term imprisonment of not less than 10 or life imprisonment, and fined not less than 50,000 yuan but not more than 500,000 yuan or confiscated property.

Legal basis: Article 196 of the Criminal Law of People's Republic of China (PRC) commits credit card fraud under any of the following circumstances. If the amount is relatively large, it shall be sentenced to fixed-term imprisonment of not more than five years or criminal detention and fined not less than 20,000 yuan but not more than 200,000 yuan; If the amount is huge or there are other serious circumstances, he shall be sentenced to fixed-term imprisonment of not less than five years but not more than ten years, and shall also be fined not less than 50,000 yuan but not more than 500,000 yuan; If the amount is especially huge or there are other especially serious circumstances, he shall be sentenced to fixed-term imprisonment of not less than 10 years or life imprisonment, and shall also be fined not less than 50,000 yuan but not more than 500,000 yuan or confiscated property:

(a) using a forged credit card, or using a credit card fraudulently obtained with a false identity certificate;

(2) Using an invalid credit card;

(3) Fraudulent use of another person's credit card;

(4) malicious overdraft.

The term "malicious overdraft" as mentioned in the preceding paragraph refers to the behavior that the cardholder overdraws beyond the prescribed limit or time limit for the purpose of illegal possession, and refuses to return it after being urged by the issuing bank.