Excel-validate total

All number amounts. column A – A1:A4= total in A6
column B B1:B4 = total in B6
In D column D1:D7 I break down numbers in A and B column to come up with the same combined total of A and B column. This total is placed in D9. What I need is some sort of validation so as to arrive at the same totals (D9= (A6+B6) and not let a person enter more or less than the total of D9 in the D column.
Confusing?