Thank you for posting here in the Community. I'd be glad to lend a hand with any questions you may have about the invoices in QuickBooks Online.
If you're trying to add a deduction on an invoice which you can't bill until the end of the contract, you can create a Retainage item and account in the system.
First, create a Retainage account by following these steps:
Once done, you can proceed with creating the item:
Now I'm adding the detailed steps on how you can withhold retainage from an invoice:
Here are some sample screenshots for your reference:
Let me know if you have additional questions with entering retention amounts. The Community is always here to ensure your success.