This is *very* basic stuff. Have you ever used a spreadsheet before? Are you
perhaps taking a class?
I am assuming that the header row is row 1, the first data row is row 2, and
the first column is column A.
The first "Budget remaining" cell needs this formula:
The second "Budget remaining" cell needs this formula:
For all additional rows, just copy the bottom "Budget remaining" cell down.
(Do *NOT* copy the top "Budget remaining" cell down.)
If there is a possibility of several things going wrong,
they will all happen at once.