Combeferre does feel like he's being underhanded by taking advantage of Valjean's politeness, but what else is he to do?

He notes the citizen, and feels even more like a scoundrel. The other man's courtesy is so accommodating and punctilious, and Combeferre is exploiting it, albeit for his own good and Enjolras's.

Nevertheless, he enters at Valjean's invitation. "Thank you. Where would you like me to put this?" Combeferre gestures at the tray.