Josh,
Move Negative Lines feature in Sales Order and Purchase Order. This is a quick way to create a Return Order or Credit Memo from an order. Here is an overview handout snip-it:

This is a quick way to create a Return Order or Credit Memo from an order.
Move Negative Lines… in Sales Order: Prepare to create a replacement sales order in a sales return process.

Move Negative Lines… in Purchase Order: Prepare to create a replacement purchase order in a purchase return process.

Note: You can add negative lines to the sales order line and the purchase order.

But if the total amount is negative, it cannot be posted.

In other words, as long as the total amount is not negative, you can post whether you have negative lines or not. Best Practice dictates when you need to post with a negative line, you should use Return Order or Credit Memo. Move Negative Lines feature is prepared for this case. Choose Action -> Functions -> Move Negative Lines… when you have negative lines.

When you move a negative sales line to your selected document type, the quantity of the line on the selected document becomes positive

Sales Return Order Lines:

The negative lines on the sales order line will be automatically deleted. (Moved)

Hope this helps.
Thanks,
Steve