How can I tie my union deduction payroll expense to the paid bill once I pay the union and clear the payroll expense?

Welcome to the Community, @lhumbles.

Tying the bill and payroll expense isn't possible in QuickBooks Online (QBO). Both of the transactions should be separated. I suggest adding a note on the Memo box to determine what the transaction is for.

Here's how you can add a note to the Memo box:
 

1. Open your transaction.
2. In the Memo box, enter a note or comment.
3. Once done, click on Save and close.
   
   

If you really need to link the entries, you can use an expense transaction type to achieve it. Simply delete the bill and record the payment directly as an expense. See the attached screenshot below:

I've included these articles to learn more about operating company expenses or bills:

• Reconcile an account in QuickBooks Online
• Enter and manage bills and bill payments in QuickBooks Online
   

Keep me updated if you have other concerns about your payroll or transactions. I'll make sure to provide any assistance required.