Increase one cell by decreasing another.

Hi there,
if I understand Your explanation correctly, You want to have formulas in A & B column, but on the other hand You want to type numbers in booth columns (once You type something in A and want to calculate in B and second time You type in B and want to calculate A). I am afraid, that there is not such a formula...
As I can see, maybe You need another column with drop down validation list to select if purchased or sold, second column with quantity You type manualy and in the trird column will be 'IF' formula to calculate stock on hand.
Also You did not mentioned what to do with max stock in column C...