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.

Try
=IF(C12="","",IF(D12+E12+F12>C12,"ERROR",D12+E12+F12))

Okay that works great!... unless I have to enter a negative number as some change orders are actually a credit. Then the sum of the cells, is assumed at zero and it gives the error message :P

Are you meaning a negative number in cell C12?

You could use the ABS() function around C12 and around D12+E12+F12 if there's a possibility they'll all be negative to compare the absolute value of the numbers.

Yeah negative in C12, what is the ABS function... sorry I am learning excel as I go. and yes if C12 was negative then DE and F would be a negative number or blank

Okay I tried this

=IF(ABS(C29)="","",IF(ABS(D29+E29+F29)>C29,"ERROR",D29+E29+F29))

with negative -500 in C29 and -200 in E29, still giving me an ERROR message.

ABS takes the absolute value of the number (basically takes the negative sign away if there is one).

Try
=IF(C29="","",IF(ABS(D29+E29+F29)>ABS(C29),"ERROR",D29+E29+F29))

ahhh I see where you are going with this brilliant

Excellent! If this fixes your problem, remember to mark the thread "Solved."

Please hit the start (*) if I've helped you!

Yep just wanted to test it, works Great, thank you for your help