if the sum of a row of cells (each cell is one month) equals a value in a separate cell (call it x) I want the formula to return 0, however if the cell is lower than that value (y) I would like it to either return a desginated minimum value or x-y; whichever is lowest.
It would seem to me that you don't need a separate condition/test for does x=y then =0, because, if x=y, then x-y=0 (within round off error, anyway).

One clarification: we are sometimes careless -- especially when talking about negative numbers -- in defining what "minimum" means. Do you mean the value closest to 0 or the value further away from 0? Mathematically, minimum means "more negative" -- meaning that MIN(-10,-5) is -10.

As I interpret your statement, you want the function to return a value between 0 and a designated minimum. The simplest statement might be something like =MAX(x-y,designated minimum). Assuming x is always less than y, this function will return 0 when x=y, x-y when x-y is between 0 and minimum, and minimum when x-y is less (more negative) than minimum.