Validation of cell based on Drop down list
 

I need to perform validation on column B depending on the selected dropdown value in column A....
I have column A with drop list whose value are
Sales Cash, Sales Credit, Purchase Cash, Purchase Credit...etc.
I want to perform validation on column B if the selection in column A is
Sales Cash, Sales Credit then column B should accept a positive value 0,
else if the selection in column A is Purchase Cash, Purchase Credit then column B should accept a negative value <0