legal ground
Article 13 of the general rules for loans
The lender shall determine the interest rate of each loan according to the upper and lower limits of the loan interest rate stipulated by the People's Bank of China, and specify it in the loan contract.
Article 14
Lenders and borrowers shall collect or pay interest on schedule according to the loan contract and relevant interest-bearing provisions of the People's Bank of China. When the loan extension period and the original term reach the new interest rate term grade, the loan interest will be charged at the new term grade interest rate from the date of extension. Penalty interest is charged for overdue loans according to regulations.