Well, first thing I want to say is you will be allotted only with MSc Math but not M.Sc math +CS. The BE branch will be allocated to u basing upon your 1st year C G.
Now coming to your question,
Yes, u can do a minor degree in finance what ever Ur branch is.. That is one of the advantages in BITS , it has such an academic flexibility.
How to apply : Every student will be getting a mail from ARC office in his/her second year first semester (2.1). Then one can go to ARC office and can apply for minor degree by filling form and get enrolled for that particular minor. One have to do 5 courses in addition to academic courses to get a minor degree.
I hope this answer is helpful.