Is it permissible to take money from one's father without his knowledge in exchange for working with him when he does not provide adequate compensation for the effort expended, especially since he promised to help with marriage preparations?

It is forbidden for the questioner to take from his father's money without his permission. The fact that his salary is lower does not justify it, unless the father forces him to accept less than a fair wage for his work, in which case he may take the difference. As for the father's bad treatment, his ill-treatment must be met with forgiveness, gentle and wise advice, patience, and prayer for him, for Allah Almighty says: "And lower to them the wing of humility out of mercy and say, 'My Lord, have mercy upon them as they brought me up when I was small.'"