Is an employee sinful if he donates his employer's money without their knowledge, and what is the expiation for that sin?

You are not permitted to dispose of your uncle's money through charity or otherwise without his permission. This is considered an encroachment upon his property. You must repent to Allah and inform your uncle of what you have done. If he approves of your action, then praise be to Allah. If he does not approve, then you must pay from your own money to compensate for what you spent from his money.