The version that I perform, and have spent much time perfecting, is much more convincing. First off, I don't use a thumb tip. For that matter, I don't use a thumb tip for any effect. Secondly, I offer a free choice of three lemons. This is a free choice and it is totally random. I let a spectator "select " a lemon via a roll of a die or by a Magician's Force. Next I write down the serial number and place it in an envelope, seal it and return it to it's owner. Upon opening the envelope, the bill has vanished only to reappear in the freely selected lemon held the entire time by another spectator. It allows free selection of the fruit, the vanish is clean, and there is no thumb tip. Plus the lemon which contains the bill at the end of the trick was held the entire time by someone else. Super clean get-a-way. Keep practicing and work on your methods a little more. I have renamed my version SourDough. Sour for the lemon, Dough for the money. As a professional illusionist, I have spent alot of time with this trick. It is a staple in every perfoamance, and it continues to blow away any audience. But if I were to do it like you presented it, my audience would likely walk out on me.