for game I need to apply a value based on an average that will change with
time. However, I need the applied value to lock as of the time it is posted,
rather than changing each time the average changes.

There is a "master" sheet which keeps a running batting average for each
player based on a fairly complex formula. There is a sheet for each game,
and the idea is to copy each player's average at that time, onto the game
sheet. however, if I just reference a cell on the master sheet, each game
value changes when the average changes. I need to lock each sheet after the
game is done.

For example if a batter has an average of .248 on game 1, I want to copy
that average from the master sheet onto worksheet 1. Four games later, if
his average has changed to .264, I want to apply .264 in worksheet 5, but I
don't want the value in sheets for games 1,2,3, or 4 to change to .264, but
remain based on his average at the time of that game.

How do I reference that cell with his average on the day of the game,
without having the values on every sheet reflect that?

TIA