Is it permissible to fulfill a vow of sacrificing two sheep by sacrificing only one sheep due to financial hardship?

A contingent vow must be fulfilled if the condition upon which it is contingent occurs. If you are unable to fulfill the entire vow, you must fulfill what you can of it, and the remainder stays binding upon you until you are able to fulfill it. It is worth noting that making a contingent vow is disliked (makrooh).