Upon seeing the merchant's reaction, Ciri will punch Nero (hard) in the shoulder and step forward to try to comfort for the poor man. She'll apologize for her friend's boorish behavior and try to reassure him that one missing pastry on a tray that looks quite full surely won't be noticed.

What kind of party is this anyway? Surely Marcellus isn't such a horrible person as to have a fine baker fired for a tiny miscount?

Depending on the merchant's response, if he continues to be distraught, she'll also declare that Nero will personally apologize to Marcellus for the misstep.