I want to know how to treat the Withholding Tax (WHT) on Quickbooks. I charge a customer a cost + 7.5% VAT, but they often subtract WHT before adding VAT and paying me.

I can help you record the withholding tax deduction on a customer invoice, Emmanuelo.

In QuickBooks, the proper method for handling customer withholding tax involves using a credit note and applying it to the invoice payment. Here are the steps you can follow:

First, set up a withholding tax expense account:

1. Go to Settings ⚙ and select Chart of accounts.
2. Click on New.
3. Choose Other Expense from the Account Type drop-down menu.
4. Select Other Expense from the Detail Type drop-down list.
5. Name the account "Withholding Tax Expense" and then click Save.

Next, set up a withholding tax expense item:

1. Navigate to Settings ⚙ and select Products & services.
2. Click on New and choose Service.
3. Name the item "Withholding Tax".
4. From the Income account dropdown menu, select the withholding tax expense account you created earlier.
5. Click Save and then close the window.

To record the withholding tax deduction, create a credit note. Then, use the withholding tax item and enter the tax amount withheld by your customer. After that, apply the credit note to the invoice payment. 

For detailed instructions, refer to steps three and four in this article:  How do I record tax withheld by my customer?

Additionally, you can utilize sales reports to gain insights into your purchases, sales, and inventory status.

If you have any further questions regarding recording withholding tax deductions on a customer invoice, feel free to ask in this thread. I'm here to assist you.