Based on this question, where the OP was asked to pay back parts of his salary in cash: I have a hard time imagining any legal reason why an employer could ask back parts of already payed salaries. Maybe if he accidentaly payed too much, or if he had granted an advance, but certainly not just because the employement contract was terminated (for whatever reason)? Even if the employee gets fired for serious missconduct, the salary is IMHO still owed, but he may later be sued for the damage caused. And none of this would happen in cash, even though paying salaries in cash is apparently still legal (but very uncommon in Germany).
Am I missing something here, or does this really look like a serious tax evasion or money laundering attempt (pay €8000 by bank transfer, note it as business expense and then get it back in cash without receipt)?