We invoice a particular client a certain amount and then they deduct a 10% fee and pay us the 90% . How/where can I post these outstanding amounts?

Welcome to the Community, @joli-cole-redbox. I'll be happy to outline the steps below so you can get through this and post the outstanding amount in your QuickBooks Online (QBO).

You can start by creating an invoice with the original amount, and then receive payment for that invoice. Here's how:

1. Access your QuickBooks Online account.
2. On the left navigational bar, click the +New button.
3. Under Customers column, select Invoice.
4. Enter the necessary details.
5. Once done, click Save.

You'll then receive the payment for that invoice you created. Refer to this article for further guidelines on how to receive payment in QBO: Record invoice payments in QuickBooks Online. 

Once done, you'll want to go to the Bank Deposit, here's how:

1. Access your QuickBooks Online account.
2. On the left navigational bar, click +New.
3. Under the Other column, select Bank deposit.
4. Select the payment that you receive from your customer.
5. In the Add funds to this deposit section, fill in the necessary details.
6. In the Amount section, input the equivalent amount of the 10% and then make it negative.

If you're unsure of what account to choose, you can reach out to your accountant for an advise. This way, they can help you choose the perfect account for this.

By doing so, QuickBooks will recognised the full amount that you receive in real life, and you won't have to worry about having any outstanding balances in your account. 

Additionally, I've got you this article in case you'd like to personalise your sales form in QuickBooks Online: Customise invoices, estimates, and sales receipts in QuickBooks Online.

You may also check this article in case want to customise a report to show specific data in your QBO account: Customise reports in QuickBooks Online.

Know that we are happy to have you here in the Community, @joli-cole-redbox. If you need help performing specific tasks in QuickBooks, feel free to post here again. Rest assured, our team of dedicated experts is always around to help you. Take care!