Something demanding.

You rack your brain to think of something convincing to say, and after a few seconds, you come up with something. You assert that you deserve more of a salary if you're going to be working under someone else, and that it's in Grillby's best interest to listen to you. You and Grillby stare each other down for a few minutes, until Grillby picks up the job application and walks away for a minute. You think that Grillby might actually be listening to you, until you see Grillby come back without anything. Grillby just crosses his arms and stares at you intimidatingly, waiting for you to make the next move. You feel like it would be in your best interest to return Grillby's pen and walk away before things escalate.

After finishing that little encounter, you doubt that Grillby would be very interested in talking to you again, though you could always try to smooth things over between you two. Aside from that, you could try talking to the green flame lady, or you could always leave and come back another time.