I's not clear, but my interpretation is that they'd called him & began the meeting with "We'd like to discuss your salary; we see your compensation is at 10%";

I's not clear, but my interpretation is that they'd called him & began the meeting with "We'd like to discuss your salary; we see your compensation is at 10%"; without making it clear that they were planning to lower it. The clerk pre-empted their plan to lower it; but tactically said "I'd not take a penny more from you" to imply that he'd assumed they were about to offer him more. By this action, he's shown that he's not going to be greedy & take more even if offered, which puts the silks in an awkward situation where to correct him & say that they meant to lower it would be rude given he'd "so generously" declined the fictional offer of a pay rise.