Help!

I am racking my brain trying to figure out how to input a dollar figure into
a cell (A1)that updates another cell (A2) with the running total (easy to do)
but the hard part is when you want to add in the next dollar figure in the
same cell (A1) but you want to increase the value in A2 by this new number.

Why do I want to do this?

I have created a spreadsheet for a game and instead of using paper money I
want to be able to increase or decrease the player's dollar amount by having
the player (knows nothing about excel) input the dollar amount they want to
either increase or decrease their running total by and I want to keep it
simple for the player so they do have to add formulas when they enter the new
dollar figure.

Hope this makes sense?