Is it permissible to take the value of a promised bonus from the employer's money without his knowledge, by submitting invoices for things that are used up, and is there a specific limit to that?

If your employment contract or its regulations clearly stipulate your entitlement to the bonus and you are unable to obtain it, then it is permissible for you to take its equivalent without the employer's knowledge, based on the permissibility of this in the issue of "al-Dhafr" (securing one's right). This is on the condition that you have confirmed that injustice has occurred to you after the project's completion and your entitlement to the end-of-service benefits. However, as long as the work has not yet finished, you are not permitted to do so; because the injustice has not yet materialized, and mere apprehension or a broken promise does not justify this action. If the employment regulations do not stipulate the payment of the bonus, then the aforementioned action is not permissible for you, even if the employer promised you that.