Is it permissible for me to receive a monthly salary from the company I founded with my friend, where I contribute capital and he contributes labor, and he receives a monthly salary in addition to sharing in the annual profits?

The default ruling is that it is not permissible for the owner of the capital or the worker to receive a monthly salary in addition to their share of the profit. If the share of either of them is set as a known amount of money, the partnership becomes invalid by the consensus of scholars. This is due to the possibility of insufficient profit, or taking a portion of the capital, or negligence in seeking profit. As for your friend, it is not permissible for him to receive a monthly salary in addition to the agreed-upon percentage of profit, because the percentage of profit is in return for his work. If he performs tasks that he is not normally obligated to perform, and that was a voluntary act, then he is not entitled to any payment. However, if he does them intending to receive payment for them, there is a difference of opinion, but the correct view is that he is not entitled to anything, because he performed work with someone else's money for which no compensation was stipulated.