Workflow Soluiton

Is a way to validate data that is on the grid when a field is changed and updates the values? I have a Quantity column and need to do some validation on it so that users cannot enter a negative value. I tried creating a custom validation on it but couldn't find a way to do any logic on that column. Ideally the way it should work is if they input a negative number, once the column updates it will give the user an error message stating that "No negative numbers are allowed" and reset it back to 0.

Sorry to ask so many questions on the forum as I am trying to finish up a hardware request form that is due next week and need to get some of the kinks worked out. In addition to the above question, I notice that if a user changes the quantity but does not click outside the box to refresh the grid and clicks the next button, an error will occur. This can be problematic as we can't rely on users to wait for the grid to refresh. Has anyone encountered this issue before and if they have, what have you done to resolve it?