Conditionally Locking Cell Values

I am trying to show the difference in taxes paid. For example if taxes on 10,000 is $500 then I change the income to 8,000 and taxes move to $300, I want to show the difference of $200. My attempt was to hold the $500 in a place holder cell then subtract the $300 later but I couldn't figure out how to hold it in place. I have only tried in excel and haven't tried any outside code like Python yet.