Adding water to milk

Q: I am a farmer and supply milk to shops. Almost all the other farmers sell their milk at a low price because they add water to the milk. The customers are aware of this as well. If I sell my milk without water, nobody will buy it because it will be more expensive than the milk with the water. If I inform my customers and mutually agree with them to add water to the milk, will it be permissible?

Breaking a vow

Q: I had made a vow that if I do a particular sin ever again in my life I will pray so many number of Rakaats. However I am finding it difficult to fulfill the vow because I keep relapsing into the sin. May Allah help me abstain.Am I allowed to change or break the vow so I no longer have to pray those many numberof rakaats?

Forgiving someone

Q: How do I know if I have forgiven someone? I had slept forgiving someone but after waking up, I again felt bad about that person, though I didn't have any bad feeling or hatred that night.