QuickBooks Online operates off of Double Entry Accounting, meaning the goal is for all transactions to balance with an opposite entry to another account.
A/R Aging Summary Example
On the A/R Aging Summary in QuickBooks Online, any invoices will be marked with a positive number (increasing the amount that is outstanding/needing to be paid on this report).
For example, the 1ne Line Agency has invoices that is overdue between 1-30 days:
Any payments from your clients will be marked with a negative value (decreasing the amount that is outstanding).
For example, James Smith has a payment that was received in the same report period, but there is no Invoice to apply it to.
How Can There Be A Negative Number?
Negative numbers will appear when where isn't an Invoice within your reporting period to apply that payment to.
For example, I ran this report as of 5/11/22.
James Smith paid me on 4/13/22 for $3,357.37.
James' Invoice Date (the Logistics Start) isn't until 5/19/22.
To say it another way:
QuickBooks Online has 'blinders' on, and is only looking for transactions and invoices within as of 5/11/22. The payment is recognized within that period, but not the Invoice date, leading to a Negative value on this customer's A/R.
However, if I extend my report dates to include the date of the invoice, I'll now see the remaining balance due by that client: