I'd like to use a formula in a cell, but let's say someone has come along and manually entered a value in that cell (and clicked 'Save'), I want to be able to revert that cell back to it's original formula without having to physically re-type the formula in the cell. Is this possible?

For example: I made a sheet with a bunch of formula's. My manager comes along and changes one of my cells (which contains a formula) to an arbitrary value, e.g. 20. He then saves the file. A few days later he realizes his input of 20 is not as good as the formula's original answer, and wants the formula to do it's job as it did before. How can he do this, without asking me to re-enter the formula manually?

I basically want to make my sheet dummy-proof. I thought of locking formula's, but that would prevent my manager from making any changes to the formula cell at all. I want the flexibility of allowing him to input values, as well as revert back to the formula, without asking me to re-enter the formula.

If this is possible - please help! Very much appreciated.