Is it permissible to change the value of a vow from slaughtering a large sheep to slaughtering a small sheep if work circumstances change and the salary decreases?

A conditional vow is disliked, but fulfilling it becomes obligatory if the condition is met. Since the condition mentioned in the question (working and receiving a salary) has not been met, fulfilling the vow is not obligatory. Purchasing a sheep and fattening it for charity is considered an independent act of charity, unrelated to the vow.