I would say no. Good tests result don't negate his bad behavior. Actions have consequences 

That's a hard one. 

This is why I think things like chores, good grades, etc. shouldn't be rewarded but expected as a minimum. Of course, you can reward every now and then - but not tell them beforehand. Just a simple "hey, I picked this up for you because you're doing so well in XYZ - keep it up" is enough. 

But in this situation, I would probably not get the toy simply due to the expultion. That's a pretty big deal, and honestly, it kind of negates the good test. 

I would probably buy him the toy, since it was promised to him, and then immediately take it away from him for the bad behavior.  He could then earn it back with good behavior.  

I would be inclined on this occasion to get him the toy, as you did promise.  However, I would then set up a chart, with a line that goes up each day for good behaviour, but down if it's bad. Then when he gets to a certain high point (usually the weekend following consistently high marks for a week)  he receives a reward. My daughter does this with her sons and it works well. 

Nope, a reward is dependent on comprehensive good behavior.  

This applies not only to kids but to adults as well.  At various times in my career I have been asked to step in and recover a foundering organization.  Invariably one of the key issues is that previous leaders have not documented performance issues within the organization and just pass problem employees off to the next leader.

In those situations one thing I do is let the organization know that there is no removal of reprimands from employee files. Far too often I have inherrited organizations where persistent behavioral and performance problems have resulted in reprimands and personnel being put on PIPs prior to being fired.  Once the employee works through the reprimand or PIP the reprimands and PIPs are removed from their file.  Almost without exception the employee returns to their historic crap.  A new leader is unaware of the history because the personnel file has no record of the historic crap.  

When I take over a struggling organization I insist on having interviews with all of the prior leaders of that organization and review each employee so that I am aware of any historic performance issues.  That way if past behavior resurects I can nail the issue to the floor right away and make sure that disciplinary action escalates if the behavior is a repeat.

So... I keep all PIPs and reprimands in the file and make sure that HR is fully aware of them and that they will not be removed ... ever.  I also add the interview notes with prior leaders to each employee's file.  Good or bad.  I want solid performers to be recognized for their solid performance even more than I want PITA behavioral and performance issues documented for all eternity.

It works with kids just as well.   "But I didn't lie this time!" or "But I did well on my test!", etc... as an effort to deflect from being sent home for crappy behavior, or any other stupid decisions,  is straight out of the toxic kid crappy behavior hand book.

With SS he always preferred me to be the primary disciplinarian.  With me he knew that consequences would be applied and then we all moved on.  He also knew that if he repeated hsitoric crappy behaviors/decisions that the consequences would deliver an escalated state of abject misery above and beyond what he experienced as a result of his previous perpetratiotn of that behavior.   With his mom he knew that consequences were long with weeks or months long recounting of the issue.

IMHO consequences should be firm, direct, and in as real time as possible.  Rewards for long term positive behavior and performance should be celebrated and durable.  Doing well on a single test is not worthy of long term reward.  Particularly when there is a history of repeated crapy behavior.