Hi @Mwiki02,
Have you made an application to renew your permit or why has this letter been sent to you if your permit is valid until August next year? What is written on the ‘Formblattantrag’?
The Negativbescheinigung is a paper you can get from Jobcenter. It confirms that you are not entitled to any social benefits from them, which is one of the requirements for having the permit 25b. However, it should be clear to the Ausländerbehörde from your payslips and rent contract whether you do or do not earn enough. They can do the calculation themselves but you can also contact your local Jobcenter and request them to issue you with the Negativbescheinigung.
Best,
Éanna