Help on stopping cells from changing

Hi
I have attached a file that I am setting up for my kids allowances. I am using an IF statement to auto enter the allowance amount every friday.
The problem is that when I want to change the allowance amount it changes all entries. I just want it to change future entries.
I know this is a simple spreadsheet but I am new to excel and just learning as I go.
Any help would be appreciated.
Thanks in advance for the help.