Is it obligatory to fulfill a vow to donate half of the pharmacy's monthly profits, knowing that in some months it is impossible to do so due to other financial obligations?

Your vow, by saying "I covenant God," is both a vow (nadhr) and an oath (yamin). You are obligated to give in charity monthly what you vowed. If you are unable to fulfill it, there is no sin upon you. It is recommended to perform an expiation for an oath (kaffarat yamin) or to pay the vowed amount whenever it becomes possible. You must give half of the profits in charity monthly whenever you are able to fulfill it. The jurisprudential schools have differed regarding the ruling on someone who made a vow but is unable to fulfill what they vowed: The Malikis hold that nothing is incumbent upon them, while others believe in the obligation of fulfilling it later. As for the Shafi'is, they hold that there is an obligation to make up (qaḍāʾ) for it without expiation for someone who vowed prayer or fasting and became unable, and that the vow is waived for the one in hardship regarding charity, with the obligation to fulfill it when they become able. The Hanbalis hold that an expiation for an oath is obligatory for someone who vowed an act of obedience but became unable to perform it due to an inability that is not expected to cease. However, if the inability is expected to cease, they must wait and perform it without expiation.