Seller changing his price for different customers

Q: We chose a cow for qurbaani. The seller is charging other customers R38 per kg. When he sent the bill to us, it was R41.50 per kg. My husband queried it and the seller responded that he has his own system of working out the cost which he does not have to explain, and we can cancel the purchase if it is not suitable to us. Is it permissible for him to conduct business in this way?

Intention when giving sadaqah

Q: 1. When giving Sadaqah should we make niyyah that I am giving this sadaqah for the good health of my children and family members? Or just the niyyah of sadaqah is sufficient in helping ease our difficulties?

2. If sadaqah is given with the niyyah of good health/protection of ones family, will one get the sawab too?

Denying Taqdeer

Q:

1. Is a person considered a Kafir if he or she does not believe in Taqdeer (Destiny)?

2. Can Dua change Taqdeer? Some scholars say that dua can change Taqdeer while others say dua cannot change Taqdeer. I am confused.

Consuming doubtful food

Q:

1. Is it true that all food is halaal until it can be proven haraam?

2. There were a few suspicious ingredients in something my mom bought that could've been haraam if processed through animals instead of plants. I chose to eat it since I didn't want to waste food or disrespect my mom by rejecting the food. Was it sinful for me to consume the doubtful food?

3. Does the hadith of not having my dua accepted for 40 days for consuming haraam food also apply here even if my dua is of repentance?

Working at massage centres

Q: I'm a Sri lankan living in Abu Dhabi, UAE. Due to some reasons I discontinued my job for last few weeks and now am jobless, but now I have got an opportunity to work at a massage center in the city with good payments and I don't know what kind of services do they provide and also I feel like it could be a prostitute center so is it ok to go and work with them? Sorry for my bad English hope you'll understand what am trying to say.