Deleting formula and replacing with value

So working with accounting I'm trying to work out a code that would freeze a cells value that had a formula in it.

Here's an example:
Person A started off at $100 salary so the forecast for future months is based off that.
After a month has passed I want that cell to be final, where the formula is no longer there or doesn't update.
So if that person received a raise all the past months value wouldn't change, only the current and future ones will.

All help is greatly appreciated.