I appreciate you for getting back to us, @square1 comp. Allow me to chime in and help you record partial prepayments to a vendor in QuickBooks Online (QBO).

In QBO, we're unable to record payments against a purchase order since it's a non-posting transaction. However, you'll need to enter a Check or an Expense affecting Accounts Payable to record partial prepayments. 

Then, convert the purchase order to a bill and apply the payments. Here's how:

1. Go to Expenses and click Bills.
2. Go to the Unpaid tab and select the bill in question.
3. Click Mark as paid.
4. In the Outstanding Transactions section, enter the amount of the partial payment in the Payment column.
5. Go to the Credits section and tick the boxes of the payments.
6. Select Save and Close.

Furthermore, run reports in QBO to track vendor transactions and balances. 

Keep in touch if you need additional assistance recording partial prepayments in QBO. I'll be available in the Community anytime to provide further help. Take care, and have a good one.