My customer deducts their commission before paying the balance on their monthly statement, how do I record this?

Hello there, @acmobiletyres,

While QBO doesn't allow creating negative deposits, you may use a service item or credit memo to record sales commissions taken in advance.

Here's how:

Service Item
 

1. Tap the Gear icon and choose Products and Services from the list.
2. Click the New button and tap on Service.
   
3. Input all the information of the commission.
4. In the Income Account section, enter an expense account.
   
5. Hit Save and Close.

Once done, you can enter the item to the sales transaction then add a negative amount. This way it deducts to the total amount due for the customer. Please see screenshot:

Credit Memo

1. Go to the Plus icon (+) and choose Credit memo.
2. Enter the customer's name including all the refund details.
3. Key in the Amount, and click on Save and close or Save and send. 
4. To apply the refund, tap the Create ⨁ icon.
5. Under Customers, select Receive Payment.
6. From the Customer ▼ drop-down, select the customer name.
7. Select the invoices from the Outstanding Transactions section.
8. Mark the credit memo under the Credits.
9. Hit Save and Close when done.

Please see this sample screenshot:

For additional information on customer credits and bad debts, I recommend the following these article: Enter and apply a credit memo or delayed credit.

If you have other questions or need help with anything in QuickBooks, drop a comment below or mention my name. It's my pleasure to help you at any time. Have a great day! Take care.