A more thorough explanation:

in diem

For each day or per day.

• The company pays its employees in diem for their daily work.
• The hotel charges $100 in diem for each night of stay.

These examples show that in diem means something is paid or charged for each day. In the first example, the employees are paid a certain amount for each day they work. In the second example, the hotel charges a fixed amount for each night of stay.