In principle, a verbal job offer is binding, as German employment law does not require this to be in writing. However, it is advisable to make sure that you have discussed and agreed on all of the above points.
This does not apply if the commitment was only made with reservations, for example if the approval of another manager is still required. It is also important that you can prove the verbal job promise. The burden of proof lies with the employee. If you have already started the job and are employed by the employer, this constitutes such proof.