Taking the 20 bucks without receiving an agreed upon good at an agreed price would be considered theft or at least expropriation and as such would not be consistent with the morality of rational self interest.
The fact of the matter is that the 20 bucks are the property of the customer and represents the portion of labor or investment that the customer engaged in in order to produce it. Keeping it without the knowledge or consent of the customer is the very definition of looting – just done so without the helpful force of government.
Honesty is actually a key and important part of rational self interest. Do not fall into the trap that many people do of assuming that rational self interest means that you do anything that favors yourself – even at the expense of others. That is NOT rational self interest or objectivism. Selfishness as Rand defined it is different from the conventional definition of the term – there are specific limits summed up in the non-agression principle.
There are no instances that I can think of where you could describe a deceitful or treacherous or villainous act as being consistent with Objectivism. There will always be a root of force or aggression which renders the act inconsistent with rational self interest.