What is required of the husband if his wife vowed to slaughter a bull and distribute it for the sake of Allah, at a value not less than 300 Omani Riyals, should she become pregnant during the month, and he finds a bull for a lesser value?

You are not obligated to anything, for the vow obligates your wife alone. This type of vow is called a "conditional vow," which is disliked but obligatory to fulfill. If your wife becomes pregnant within this month, she must fulfill her vow by slaughtering a bull whose value is not less than the mentioned amount, in accordance with the Almighty's saying: "And let them fulfill their vows," and the Prophet's (peace be upon him) saying: "Whoever vows to obey Allah, let him obey Him; and whoever vows to disobey Allah, let him not disobey Him." If the value of the bull is equal to what your wife vowed, then it suffices her; but if it is less, then it does not suffice her.