Is there a word for this act?

Let me give you an example where I want this "mystery" word to fit. There are two person "A" and person "B". A had installed an air conditioner in his house long ago and has called in the customer care services for servicing it. B is the technician who went to A's house for troubleshooting. As per the rules, A is supposed to show the purchase bill to the technician after he finishes his job. But A had kept the bill in his office and even though A knew this was mandatory, he still did not bother to bring the bill because he thought that once the job is done, it cannot be reverted back to it's undone state and B would have no choice but to return back to the station with disappointment and anger because of the customer satisfaction and user experience policies which requires him to be polite and friendly to the customers. So he(A) is reluctant to bring the bill and he doesn't care about B. He just wants AC to get fixed. Mind you, A is not lazy.

So, the sample sentence is:

This was an act of _______ on the part of A.

One word that springs from my mind is "unjust". I am not sure though. Other ways to describe the situation in a sentence?

I would say that person A is exploiting or taking advantage of a problem with the rules. The fact that the rule does not require presentation of the bill before work starts makes it very difficult and costly to enforce. Basically, you'd have to undo the work you already did to enforce it. Knowing that this is a weakness in the rule, person A simply exploits this weakness for personal convenience.

In this case the sentence might be worded "This was an act of exploitation on the part of A" but that doesn't sound very natural. I would probably word it differently, something like "This was an exploitation of the rules by A" or possibly "A has abused the rules."

If the action was knowing and intentional, it might be considered misfeasance or malfeasance, thought this is generally used more when referring to someone acting in a professional capacity, such as a corporate officer or government official.

Also, as with negligence, these words have specific legal meanings, so could be interpreted as legal threats.

An appropriate term for this might be manipulation. You can, of course, expand that to indicate the nature of manipulation.

This was an act of manipulation through intentional negligence on the part of A.

A is manipulating the process. If this happens enough times, B's company will eventually require technicians to have the bill in their hands before they perform service. Sometimes, A will be charged for the visit even if B performs no service because A did not have the necessary documentation.