Message box when a formula becomes negative

Hey,

I hope someone can help me!

I want a error message (or even better a prevention mechanism) to warn me when a cell becomes negative.

I have four worksheets, "Stock" "Purchases" "Sales" and "Stock Check"
In "Stock Check", ("Stock" Cell E8 + "Purchases" Cell G8) - "Sales" Cell G8 = "Stock Check" Cell I8


"Stock Check" Cell I8 should ALWAYS be greater or equal to 0!

I want an error message to pop up if it ever falls below zero, (or better still not allow Cells in Stock, Purchases and Sales to be changed if it causes "Stock Check" Cell I8 to become negative)

I hope someone can help me, it would be really appreciated!