Answer:
Since this is an example of the colliding task of two different managers, you would have to explain the point of view of the upper level manager and what he/she told you was your duty in the first place.
Even if the assistant manager thinks that not entering the sales is the most efficient idea at the moment, it would be wise to stick to the upper level manager's requirement.
The second course of action would approve the assistant manager's requirement. However, you would have to put out to your upper level manager, in a straightforward manner, that you listened to the assistant manager's suggestion. This is the course of action to opt for if you strongly think that the idea of not entering the sales is correct and won't cause damage that you and the assistant manager will be liable for afterwards.