Your customer would Ike the Reward team to be able to override the final salary...

Create a custom editable Money field and use custom validation to check that values are not entered until the correct date

Create a custom read-only Money field and change read-only to No on the correct date.

Create a custom editable Money field In the formula to calculate the final salary, use the dateDIffO function to determine if the custom column can override the calculated value

Create a custom editable Money field with field-based permissions set to read-only Change the permissions to editable on the correct date