QuickBooks Online operates off of Double Entry Accounting, meaning the goal is for all transactions to balance with an opposite entry to another account.

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.

Negative numbers will appear when there 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 that period 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: