Unable to fulfill a vow of freeing a slave
10-8-2015 | IslamWeb
Question:
I vowed to free a slave and fast three days. Is it permissible to delay the fulfillment of the vow until I obtain the money necessary for freeing a slave?
Answer:
All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
It is known that fulfilling a vow of freeing a slave is something unfeasible at this time, given that there are no slaves. Consequently, you are not required to fulfill your vow. The Shaafi‘i scholars 
stated that if a person vowed to free a certain slave and then he (the slave) died or was killed, then the maker of the vow is not required to free another slave instead of the original one. [Hashiyat Al-Jamal]
However, it is more cautious to offer the expiation for breaking an oath. The Hanbali scholars stated that expiation is due when one is unable to fulfill his vow. In Al-Mughni, Ibn Qudamah 
related from Ahmad regarding a person who vowed to free a certain slave and the slave died before fulfilling the vow:
He is required to offer expiation for breaking an oath. He is not required to free another slave, because it is something that he missed, based on the Hadeeth narrated on the authority of ‘Uqbah ibn ‘Amir 
. This is my opinion regarding a vow wherein the person missed the opportunity to fulfill it or one that he is unable to do.
The Hadeeth narrated on the authority of ‘Uqbah ibn ‘Amir was reported by Muslim, Ahmad, At-Tirmithi and others . The Prophet, sallallaahu ‘alayhi wa sallam, said:
• "The expiation of a vow is the same as the expiation of an oath." [Muslim]
• "The expiation of an unspecified vow is the same as the expiation of an oath." [At-Tirmithi]
Allaah Knows best.