Ad-valorem goods need a base to apply the percentage to, and that base is not simply your selling price. The excise price is the higher of the designated retail price and the actual retail selling price, taken on the date of the taxable event. Two prices go in, the larger one comes out, and it is resolved per product per date rather than once and for all.

The click path. Open Accounting → Configuration → UAE Excise → Excise Price Register, again a Manager menu. Entries are dated and held per product, so one product accumulates a history of prices as the market moves. When a declaration line is computed, the module reads the entry in force on the taxable-event date on that declaration — not today's entry, and not the newest one on file.

That date behaviour deserves a sentence of its own, because it is where good intentions cause damage. If you correct a price by editing an existing entry rather than adding a new dated one, you have not changed the price from today; you have changed the answer for every date that entry covers, including periods you have already filed. Treat the register the way you would treat a rate table in a ledger: prices change forward, by a new dated entry, and the old entry stays where it is as the record of what was true then.

Read those two rows together and the rule becomes intuitive. In January the market price is above the designated price, so the market price is the base. In April a promotion drops the market price below the designated price, and the designated price holds the floor — a discount does not reduce excise. Falcon Energy's January release of 7,200 units therefore carries 7,200 × 9.50, which is AED 68,400.00, and that is the energy-drinks figure on the January return in Lesson 8.

Now price the omission. Suppose only the designated retail price of 8.00 had been recorded, because that is the number the supplier sent and nobody checked the shelf. The base resolves to 8.00, the same release computes 7,200 × 8.00, which is AED 57,600.00, and the liability is understated by AED 10,800.00 in one month on one product. Nothing on screen looks wrong. The register is populated, the declaration confirms, the return prints.

The failure mode: no entry in force on the event date. Where an ad-valorem product has no register entry effective on or before the taxable-event date, the price resolves to zero and the declaration line computes to zero. The fix is to add a dated entry effective on or before the event date, then reset the declaration to draft and confirm it again so the line picks the price up. Build the habit of adding the register entry at the same moment you tick Excise Good on a new product, because the gap between the two is exactly where zero-value lines are born.