What should I do about the money I took from my employer without his knowledge, and does the claim for it become void after he forgave me without knowing what I took, especially since I do not have the money to repay it now?

You must repent to Allah and strive to return this money to its owner. If you are unable to do so, and he does not forgive you for it, it remains a debt upon you until you are able to repay it. It is permissible for you to deduct what is owed to you from him from what you owe him, if he has not forgiven you for what is confirmed to be owed to you by him, such as money for working more hours than agreed upon, or a salary increase that was agreed upon but not fulfilled.