Vowed Not to Eat Whey
24-3-2019 | IslamWeb
Question:
Previously I learnt that majority of scholars held whey is haram. Then due to this I made a vow to Allah swt to avoid it. but now Ive realised its in so many of my favourite foods so can i start eating it again and expiate for it? Salam
Answer:
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad 
is His slave and Messenger.
To begin with, we inform you that we do not know the whey which you claimed that most scholars are of the view that it is forbidden, and we did not find any mention of it in the Fataawa of the scholars.
In principle, all kinds of foods are lawful and not forbidden. Milk and dairy products are among these lawful things, so it cannot not be ruled to be forbidden without a sound Islamic evidence proving its prohibition.
What we can say about your vow is that if whey is lawful (to consume) -and this is the principle rule-then abandoning your vow is not effective because it is making a vow to abandon a lawful matter. This kind of vow is non-consequential and it does not necessitate anything according to the majority of the scholars, because it is not a vow to do something by virtue of which one draws closer to Allah.
The Prophet said: “A vow is (valid) only when one seeks the Pleasure of Allah.” [Ahmad and others, and Al-Albaani and Shu’ayb Al-Arna’oot classified as sound]
Nonetheless, some scholars held the view that in regard to the vow about what is lawful, the person has the choice either to fulfill it or to expiate the same way one would expiate for breaking an oath.
Therefore, if you want to consume that whey and it is lawful, then in order to be on the safe side, it is better to expiate for breaking your vow (like you would expiate for breaking an oath).
If you discover that this whey is forbidden, then abandoning you will be abandoning what is forbidden, and this also non-consequential. In this case (if it is forbidden) it is an obligation upon you to abandon it whether or not you have made a vow to this effect. In which case, it is not permissible for you to consume it whether it is pure or mixed (with other food).
For more benefit, please refer to Fatwa 198295.
Allah knows best.