Could anyone help..?

I have a work sheet with data validation drop down list for user selection,
i also have a formula in the same cell were data is sometimes inserted
without the user having to make a selection from the list, this is fine,
users can and need to insert other data which is not in the drop down list,
no problem here..

Is there any way to allow this data to change or be deleted but still retain
the formula, in the cell if the cell were blank?

Any help would be appreciated