Question: Once I vowed that if my relative becomes completely cured from his illness, then I will give a cow to sadaqah. Unfortunately he passed away. Now do I need to fulfil this vow?

Answer: As per your question, your relative did not get cured rather he died thus, the condition for fulfilling this vow did not meet. Therefore, it is not obligatory to fulfil this vow. However, it is entirely up to you to make a sadaqah as isal-e sawab (convey the sawab) for the deceased by giving to poor, orphans or really needy ones.

References: [1] Fatwa Shami vol-3, p-735