Your unpaid Sales Invoice for 100 is stored as an Accounts Receivable Asset - money not received
Usually if you are paid 90 of the 100, then the unpaid 10 remains as an Accounts Receivable Asset
But in this case the 10 is paid via the customers Withholding Tax (WHT) deduction so the 10 gets transferred from the Accounts Receivable Asset account to the WHT Asset account as it is no longer owing to you by the Customer, but it is now owing to you by your tax authorities as an offset - tax paid by you but indirectly.
No, the WHT is now recognised in the period it was created / deducted by the Customer. Until the Customer actually pays you the WHT itself doesn’t exist, it’s just a future obligation which will occur when the Customer payment is made. If they never pay you - there would be no WHT as it is their obligation to create it upon payment, it is not yours to create upon invoicing.
Whenever you declare / claim that WHT as “paid” on a tax authority form.
a) If its on a quarterly return where you have collect 450 VAT, paid 325 VAT and 10 WHT
Your payment would be 115 made up of 125 to the VAT account and -10 to the WHT account
b) If its on an annual income return and the tax on your profit is 1000, but because of the withholding tax of 10, you only owe 990, at this point the withholding tax account balance is cleared to an Equity account.