Welcome back, @golden2010.

Let me guide you through the process of handling prepayment in QuickBooks Online (QBO).

When you receive a prepayment higher than the invoice amount, you'll need to create a sales receipt. This will increase the amount in your liability account to show that the money isn't truly yours yet, and to avoid treating it as income until later. Here's how:

1. Click the + New button, then select Sales Receipt.
2. Select the customer from the drop-down.
3. Choose the bank account from the Deposit to drop-down.
4. Select the Retainer item you've set up in the Product/Service column.
5. In the Amount column, enter the (300) amount.
6. Click Save and close.

Once done, you need to turn the retainer into credits on the invoice. This way, it'll decrease the amount in your liability account and applies the credit to your customer's invoice, turning it into income. Please be guided with the steps below:

1. Click the + New button, then select Invoice.
2. Select the customer from the drop-down.
3. In the Product/Service field, select the services or goods you provided and enter the (100) amount.
4. On the last line of the Product/Service field, choose the Retainer item. Then, enter the (-100) amount as a negative to subtract it from the invoice.
5. Click Save and close.

I'm adding this article for the detailed steps: Record a retainer or deposit.

You might want to review all your sales transactions made in QuickBooks Online. You can go to the Sales menu and select All Sales.

Please know that I'm just a reply away if you need any further assistance recording prepayments from your customers. Wishing you continued success, @golden2010.