Can the one appointed to purchase make a profit?

Q: I need Shar'ee guidance regarding a certain trading transaction:

My relative asked me to buy a washing machine for him. The washing machine cost Rs.3000 for example.

1. Can I inform my relative that the machine cost Rs.5,000? He does not know that I will charge him extra. 

2. Can I first inform my relative that I started selling washing machines and then charge him Rs.5,000?

Getting married under the age of 18

Q: In Islam, is it halaal to get married under 18 but by Law its illegal to get married under 18?

My question is: Can I do nikah but not get married by Law (I'm under 18 and the girl is under 18 too and we are in Europe... In my country like most of the countries its illegal to get married under 18)

Ma Sha Allah im 17 and she is 16 which means we have to wait 2 years... We both are muslims Ma Sha Allah, and she has a "wali" and everything else. Alhamdulilah its okay but that is the one and only problem.

Unmarried woman taking an oath that if she does something then her nikaah will break

Q:

1. If before giving the marriage consent in front of witnesses a girl takes an oath that if she does a certain thing her nikaah is invalid but she breaks the above oath when she is married ...is her nikaah valid?

2. If after giving the marriage consent in front of witnesses a girl takes an oath that if she does a certain thing her nikaah is invalid but she breaks the above oath when she is married ...is her nikaah valid?

Seeing a white spot on one's underwear

Q: Sometimes when I go to the bathroom I see a white spot/stain. The thing is that the stain is there when I am awake. I mean that if I don't sleep for a whole day and then see it is there. Even If I don't get aroused there is a spot. It doesn't feel wet and neither does my private part fell sticky or slimy like its covered nether does my skin. This case is when I haven't slept for whole day after changing underwear in morning. I don't even feel anything if it there or while coming out if something does come out

1. Am I impure?

2. Do I have to change my underwear and do I have to do ghusul?