If you want to manage negative salaries, i.e. you want them to be carried over to the following month, you need two specific salary items.
The first one is a "Negative salary to be carried forward" type field and the second one is a "Negative salary carried forward" type field. See here if they are not standard in the list of available items and if you need to create them.
He was given an advance of 350.00 and his net salary is now -37.85. The program sets the net to 0.- and stores the 37.85 to be deducted for the next month.
Next month :
The program automatically catches up with the amount. Even if the person does not have enough salary, it will try to do so until it gets there by carrying over the amount from month to month.