So bounce the selected value against a vlookup from the same index you are populating the combobox. In other words, sheet1 has the following:
The combo box gets its values from range A1:A2. The combobox places the selected value into D1.
Now, do a vlookup against the posted value. So, if your combobox places the value of apple into cell D1 then the vlookup formula would be as follows:
Lookup on range A1:B2 for the value in cell D1, and dont worry about matching the value exaclty (false).
I hope this helps.