Is informing the employer about the manipulation by employees who transferred office transactions for the benefit of another person considered a fitna (strife/discord), and does it entail sin? And am I obligated to inform him?

The default is to conceal the mistakes of an employee and advise him secretly. If he accepts, then all praise is due to Allah. If he does not accept, and the harm caused by his actions can only be removed by informing the employer, then there is no objection to that. In fact, it may be obligatory if no greater harm is feared from it. This is considered advice to the employer and not forbidden backbiting.