Will I be sinful if I reduce my maid's wages after I vowed to increase them, given that she did not work for half a month, and is it permissible to agree with her to work for half the period for half the wages?

Undertaking a conditional vow (nadhr mu'allaq) is disliked, because it does not avert anything; rather, it is a means to extract from a miserly person. However, it is obligatory to fulfill it if the conditioned event occurs, based on the saying of the Prophet, peace and blessings be upon him: "Whoever vows to obey Allah, let him obey Him."

Therefore, since the promotion has occurred, you must increase the monthly salary as you vowed, unless you had another intention. As for employing the maid for half the duration at half the wage, this does not contradict the fulfillment of the vow. Your vow to increase the maid's monthly salary to (1500) dirhams does not necessitate that you must employ her for life.