All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
A vow is an offering that a person commits himself to present to Allah if a given good thing happens to him or a given bad thing doesn`t hit him, and it isn`t obligatory in Sharia. However, if the conditions of the vow are met, then it becomes binding because Allah The Almighty said: "Then let them complete the rites prescribed for them, perform their vows.."{Al-Hajj/29}. Moreover, the Prophet(PBUH) said: "He who vows to obey Allah, should obey Him. But he who vows to disobey Allah, should not disobey Him."{Bukhari}.
Since your vow has met its conditions, then it is incumbent upon you to fulfill it whenever you can because you will remain liable for it. However, there is no harm on you if you delay fulfilling your vow until your financial conditions improve. Moreover, it isn`t a condition that you perform your vow in your country if you haven`t specified doing so.
Therefore, you can authorize someone from another country to sacrifice a sheep on your behalf if its price is cheaper there. This is because fulfilling a vow isn`t restricted to a particular place unless the person who took the vow had specified one. Al-Ansari, a Shafii scholar, said: "It is permissible to perform a vow, a bequest, and an expiation in another country, if the person taking the vow haven`t specified a certain place for fulfilling any."{Asna Al-Matalib, 1/531}. And Allah knows best.