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

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

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

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