Excel Formula Help!!

Hi, 

I'm looking to write a formula that can help my class with some basic budgeting skills. They basic layout is each column is it's own category with an allotted amount for each: ex column A is for groceries with a total of $90 to be spent in that pay period. Each column has a sum cell (B31) so students can see how much they have spent and right beneath it in the same column, there is a cell that lets them know how much of the allotted amount for that category they have left. I was wondering if there was a way to modify it so that in the cell (ex: B 32) that currently displays the remaining balance for that category if gone over budget (as in the number is now in the negatives) could be stopped at zero and the negative balance could be subtracted equally from other categories?

To visualize what I've done so far: 

The formulas for row 13 are =sum(ColumnLetter2:CL12)

Row 14 is where it differs by each cell;

A: =minus(90,A13)

B: =minus(100,B13)

C:=minus(120,C13)

D:=minus(50,D13)

E:=minus(40,E13)

F:=minus(60,F13)

G:=minus(90,G13)

H:=minus(75,H13)

I:=minus(50,I13)

The main question comes once students start filling in the spreadsheet. With the hypothetical fill of : 

How can I create a formula in row 14 that continues to show remaining balance, but only when a column reaches negative (like the beauty) takes the negative value (15 dollars) and equally subtracts (1.875) it from the remaining columns that can have that amount subtracted without going negative themselves? I wouldn't want any of the overspent value of beauty to be taken from Misc. as there is  only one dollar left and that is less than the evenly split amount (1.875). Is there a way to write such a formula into the formula for row 14 or would it need to occur in a different row? I'd love for my students to be able to see how to manage the occasional sometimes necessary overspending in one category to see how to adjust spending in others (hopefully it wouldn't be for beauty lol). I've thought about using If formulas, but am struggling on how to fit in all the criteria. I would appreciate any help in creating a formula!!

Thank you so much!!!