Trying to get a value to remain copy into a cell and remain the same when condition met

I have an A/R spreadsheet I'm working on that needs to be updated constantly. I've searched around, but I'm not sure I'm unable to load the file for privacy reasons. In essence, here is what I'm looking at:

T73 is calculating the amount Due from multiple clients per other formulas.

the following cells in the row are by Month. U73 = amount due for January, V73 = amount due for February, and so on. These stay blank until "Final" is entered.

What I need is that once U72 says "Final", that U73 = the amount of T73 and not change again. So that when V72 says Final, V73, the February number, will = T73 and then not change. T73 will be constantly changing as numbers change, but I want to 'freeze' those numbers at the end of the month. (but it can't be done by a calendar as it depends on when the numbers are added in.)

(Edited: Oh wow. Sorry about the weird header. I just re-read that. That's what I get for trying to do something like this while on the phone!)