A defence company sales manager discovers that his division manager has built a bribery mechanism inside the company and he needs to stop them before they will be able to fire him

I am not sure the stakes are high enough.

A defense company would sell weapons, perhaps even bombs, helicopters, assault weapons, military drones.

Why would they take bribes?
To make money selling guns to organizations on the terrorist watch list.

In other words, if an unscrupulous organization bribes the division manager, his boss has a secret mechanism to discretely sell weapons to terrorist organizations. The Bad guys.

Now the lead discovers what’s going on.

He thought he worked for the good guys, but instead, he works for the very people supplying terrorists.

The story would be what the lead character does about it. What actions the lead takes to stop his manager.

It could be the lead risks his life gathering evidence in order to go to the authorities and become a whistleblower.

But the stakes should be higher than the lead just worrying he will lose his job.

Just some thoughts,
Hope this helps.