Most likely, you're reading this because you saw a negative number in the Assigned column, and you're asking yourself: How did that get there? Fortunately, you've come to the right place!
This typically happens when a category has cash left over from a previous budget month, which gets moved to a different category in the current month. Let's walk through an example:
Cash Left Over
When you select any single category in YNAB, in the right sidebar (or Inspector), you'll see if there was any cash left over from the previous month. Even though it doesn't have its own column in your budget, that leftover money is part of the equation that results in the amount Available for that category.
Here I had $54 left over in the Electric category at the end of May, which rolled into June—and I decided not to assign any more money (I got a credit on my electricity bill this month and don't owe anything), leaving Assigned at $0.
In fact, I realize I need extra money in my Gifts category for June (lots o' birthdays). I move $34 from Electric to put there, knowing I'll still have $20 left Available.
Same as any time money is moved out of a category, the Assigned value will decrease by that amount. Since $0 was assigned in Electric this month, it will decrease to negative $34, whereas the amount assigned in the Gifts category will increase from $280 to $314.
Back in the right sidebar of the Electric category, the full equation is visible:
$54 Cash Left Over From May
-$34 Assigned This Month
$20 Available This Month
As long as the Available amount is still green, it's all good—there's no reason to worry about that negative number! If it turned yellow or red though, you may want to move some money back.
Budgeting in the Future?
While negative Assigned amounts in the current month are just fine, it should always be avoided in any future months. If the money ends up being spent in the current month, it can cause some unintended overspending at the start of the new month. When assigning or moving money in a future month, be sure that the Assigned column never dips below zero!