Fatwas on Water

Summarized Fatawaa

I`m a cashier at a branch of the Civil Service Consumer Corporation. Sometimes, customers pay more than the due amount. Do I have the right to take that extra money knowing that I cover shortages from my own salary?

All perfect praise be to Allah the Lord of the Worlds. May Allah`s peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
If customers give that money willingly to help you with the potential loss then this is permissible. And Allah The Almighty Knows Best.

Should missed Sunnah prayer be made up?

Making up missed voluntary acts of worship is from Sunnah.

What is the ruling on someone who perform their prayer after finishing it because they believe they missed a Rak'a, a prostration, or that they did not perform it correctly (thinking their prayer was invalid)?

If he was sure that his prayer is void then reperforming it is an obligation along with figuring out the reason of invalidity so long as this wasn't out of uncertainty. And Allah Knows Best. 
 

In our region in southern Oman, specifically in the city of Taqah, there is a mosque that they say was built hundreds of years ago. This mosque is surrounded by graves, meaning it is in the middle and the graves are around it. People in our area differ, with some considering it permissible and others considering it forbidden to pray there. Moreover, my grandmother saw in a dream that she entered this mosque..."
 

Praying in this mosque is valid and and there is reprehensibility in its location so long as no grave exist within it. Besides, Sharia rulings aren't derived out of dreams. And Allah Knows Best.