Is it permissible to accept a tip given with the intention of charity on behalf of the deceased's soul at mourning houses without the knowledge of the employer, given that the agreement stipulates that tips belong to the employer, and that his knowledge of it might lead to my dismissal from work?

If there is an agreement between you and the company that tips belong to the employer, then you are not allowed to take them without his permission, due to the obligation to fulfill conditions. The original ruling concerning tips given to workers is that they are not theirs except with the employer's permission. Therefore, you must excuse yourself from accepting such tips from the outset, and if you do take them, hand them over to the employer. This is based on the Prophet's (peace be upon him) saying: "Whoever among you works for us, and conceals from us a needle or more, it will be a ghul (unlawful gain) that he brings on the Day of Resurrection."