I'll describe a situation that might sound random and sudden, but I'm sure we keep seeing now and then; I'd like to know if there's any suitable adjective that you can think of. 'Oily' and 'smarmy' were suggested by a friend, but perhaps there is a better word?
Say I donate an expensive machine to a nearby grocery shop, or I perform some huge and helpful favour that really makes them indebted to me. They, out of gratitude, offer to keep giving me free products. I insist otherwise, that they mustn't be so conscious of me, and that I insist on paying each time. One day I buy something and don't have the whole amount. I sweetly request them if it's okay by them if I pay the remainder in the evening. They know I'm good for it, it's obviously okay with them, a ridiculous request perhaps, and even I know that they'll say yes. Nevertheless I have a polite and humble way of asking for the credit. It's not a genuine query, but I still make it sound like I'm earnestly requesting.
Any adjective for the forced humility/politeness in such a situation where one knows one can't be refused? Perhaps similar 'benefit of doubt' is displayed by superstars — say, when Tom Cruise walks up to a hotel rece and says "Excuse me. Hi, my name is Tom Cruise..."