The interests of the manufacturer in an audit is to verify the software used, i.e. to check the licensing. The legal situation is clear-cut and the rulings of the European Court of Justice and the Federal Court of Justice of Germany superimpose the manufacturers’ own conditions and clauses. The manufacturers are also aware of this.
A reference to the valid legal situation should suffice in eliminating any objections. You are even able to entirely object to an auditing of your used software. That is to say, the exhaustion of intellectual property rights also applies here: Sold is sold; the manufacturer no longer has the right to verify used software.