My situation was to find a way to add an old Bill that was already paid far in the past (by direct debit), to a current Project in QB in order to see the project costs. I merely wanted to mark the Bill as paid, without affecting our bank account.

We could not link the Online banking payment to the Bill because those bank transactions were older than 90 days and could not be linked to QB (we recently started using QB). I tried everything, but no check or credit note payment method would prevent the Cash and Cash Equivalents Account from decreasing, however I needed the Project Expense Account to increase in order to show that those Materials-Cost of Sales were part of the Project expenditure. So finally, this is what I discovered that works:

QB online:

Click New

Click Bill

In Category details enter a one-liner with the Description as the Supplier Bill Number. Then enter the total bill amount as a negative amount.

Now, in Item details, enter all the products/services for the invoice as the actual amounts.

Make the products in item details Billable, so that the Project can see those expenditures and they can be taken into account when billing the Customer for the Project.

Save and Close, and, voila! Balance is 0 and status is Paid, and you have a record of the billable items, and a record of the Bill.

Hoooray!