Yes.
According to case law, the parties to a contract may agree upon minimum and maximum weekly working time.
Examples
The parties may stipulate that the working time in each week is to be between 30 and 40 hours.
In such a case, the Court has held that the normal working time agreed between the parties was a minimum of 30 hours per week and that the employer was required to ensure that the employee was assigned work and also remuneration corresponding to that minimum working time, during which the employee is deemed to be in the service of the employer.
An indication in the employment contract of a minimum/maximum number of weekly hours instead of an indication of a fixed weekly working time is not penalised, in particular by conversion of the contract into a full-time contract.
The result of such a clause is that the employer may decide to raise the working time up to 40 hours per week without those hours being regarded as overtime.
However, clauses providing for an excessive difference between minimum hours and maximum hours are not permissible.
Clauses providing for permanent availability of an employee are also to be regarded as void.