I want to share some insights about lining a purchase order to a paid bill in QuickBooks Online.
At this time, being able to link a purchase order to a paid bill On QuickBooks Online isn't an option for us. The bill must be open so you'll be able to combine it with the PO. However, if your transactions are not reconciled yet, you have the option to delete the payment and link the PO successfully.
On the other hand, to ensure the best course of action for your organization, I'd suggest reaching out to your account.
I'd also encourage you to visit our QuickBooks Online Blog regularly to keep updated with our latest news and updates including product improvements.
Please keep in touch with me here should you have any additional questions or concerns, I'm always around to talk QuickBooks. Thanks for coming to the Community.