What are some limitations that could occur during onboarding in Germany?

When onboarding an employee in Germany: 

• We cannot backdate employment agreements.
• Under the Employment Agency model (AÜG (Arbeitnehmerüberlassungsgesetz), we cannot assign an employee to the same client in the same position for a period longer than 18 months.
• We do not offer internships at the moment.
• We do not offer hourly rates, just monthly salary.
• We cannot add a probation period of more than six months to the employment agreement. 
• If an employee is not a board member or managing director, we cannot contractually agree that overtime of over 10–20% of the regular working time shall be deemed as compensated by the regular remuneration.