Is it permissible to take from funds designated for specific tasks with the intention of repaying them later, especially when in dire need of money, knowing that the merchant has promised a large sum in the future?

What you are doing falls under the category of betraying a trust. It is not permissible for you to take from your partner's money without his knowledge. Your shyness in demanding your right does not justify what you are doing. Therefore, fear Allah and repent to Him.