If a person makes a vow to sacrifice a sheep, for example, if Allah saves his relative, this is a vow to do an act of worship which must be fulfilled. If your mother died before fulfilling her vow, then it is regarded as a debt that she owes and must be paid for out of her estate. If she did not have any wealth, it is mustahabb for her children to do it on her behalf. Al-Bukhaari (2761) and Muslim (1638) narrated from Ibn Abbaas (may Allah be pleased with him) that Sad ibn Ubaadah (may Allah be pleased with him) asked the Messenger of Allah (blessings and peace of Allah be upon him): My mother died, and had made a vow (that she did not fulfil). He said: Fulfil it on her behalf.
An-Nawawi (may Allah have mercy on him) said in Sharh Muslim: The words of the Prophet (blessings and peace of Allah be upon him), Fulfil it on her behalf indicate that obligations that were required of the deceased should be fulfilled. With regard to financial obligations, there is scholarly consensus (that the must be fulfilled). As for obligations to do other acts of worship, there is a difference of scholarly opinions concerning them.
Moreover, the view of ash-Shaafai and others is that financial obligations that were required of the deceased, such as zakaah, expiation and vows, must be fulfilled, whether he left instructions to that effect or not, as is the case with debts owed to people.
It should be noted that our view and that of the majority is that the heir is not obliged to fulfil the vow that was obligatory upon the deceased if it was not financial, or if it was financial but he did not leave behind any estate, but it is mustahabb for him to do that. End quote.
If a person vows to sacrifice a sheep or the like, it is obligatory for him to fulfil the vow, and it is not acceptable to pay the cash value instead. The Standing Committee for Issuing Fatwas were asked about a woman who vowed to sacrifice a lamb every month, but she found it difficult to buy lambs, slaughter them and distribute the meat. Is it permissible for her to give the cash value of the lambs?
There is no hardship in sacrificing lambs; rather she has to continue sacrificing them and distributing the meat to the poor, and it is not acceptable for her to give the cash value, because the Prophet (blessings and peace of Allah be upon him) said: Whoever makes a vow to do an act of obedience to Allah, let him do it, and whoever makes a vow to do an act of disobedience to Allah, let him not do it. End quote.
Shaykh Abd al-Azeez ibn Baaz, Shaykh Abdullah ibn Ghadyaan, Shaykh Abdullah ibn Qaood.
Fataawa al-Lajnah ad-Daa'imah, 23/395
Based on that, it is not permissible to give the cash value instead of slaughtering the sheep; the fact that the questioner lives in Europe does not mean that she cannot delegate someone to sacrifice a sheep and distribute the meat to neighbours and friends, either in the country where she is living or elsewhere.
And Allah knows best.
Reproduced from Islam QA