Is it permissible to deduct amounts from a servant's salary to motivate him to work hard, with a pledge to return them to him in the future?

It is not permissible to withhold or delay the wages of a servant or employee. Rather, it is obligatory to pay their due when the payment is due, because the money is not yours to dispose of as you wish. Deducting their wages—even with the intention of returning them—is considered a delay (in payment), which is an injustice and an aggression. It would have been possible to motivate them by increasing their salary or paying their salaries ahead of time.