Sum of cells (DEF) to not exeed the value of C, Need formula

Here is a billing form I have been working on for a while, I am really happy with it, I just have one last piece I wanted to try and add but I don't know if it is possible. if you look at AIA 2 you'll see columns labeled C,D,E,F,G.

Column C is the contract amount
Column D is what was paid on the contract last month
Column E&F are what is being requested this month

What I would like is if I could somehow have excel recognize if the added amounts in D,E,& F are greater than the amount in Column C, and if it is display "ERROR" or something like that.

Here is where it gets a little harder. I will be sending this form to a lot of my sub contractors once its complete. I need them only to be able to enter a number in that cell, not be able to edit the function, because they will accidently delete something and I'll be getting phone calls all day, and messed up forms.

I have locked and hidden ever other cell except these, as these cells are the only areas in which they need to input data. Hoping for a little forum magic!

I tried this in cell D12 but it wouldn't take it.
=IF(SUM(D12:F12)>C12,”ERROR”,D12,)

I also tried using data validation on the G column, since the G column is the sum of DEF (Whole number less then or equal to C12) , but it only works if I open the G cell to edit and then hit enter. Otherwise it just allows it to run over the amount.