Snapshot Field From Same Table

Looking for a way to create a snapshot field on a field in the same table.

Example:  I add a sale and a field generates a fee (using a formula field).  This fee may change if the total sale amount changes.  But if I make a change to the fees formula, I want the old value to stay.

So update the snapshot field if the sale price changes but keep it the same if the formula values are changed.

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GH GH
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