I am calculating 24% vat for Iceland, no matter how I make the calculation, it turns out with incorrect calculation. Manager is calculating 9.251,00 when the correct amount is 9.252,00. It is not good when I have a purchase order from the customer and our amounts do not match.
This behavior is a result of the fact that since September 2002, regulations in Iceland have required that amounts on invoices be stated in whole krónur only. Manager (as well as almost all other accounting software) calculates and applies taxes on a line item by line item basis, adding the line item amounts to obtain totals.
In your case, @olafsg, it coincidentally happened that all tax amounts were less than half a króna, so all were rounded down. Had you used a different currency, or no currency as @Brucanna suggested, this would not have happened.
If you search the forum, you will see that this line-item rounding behavior on taxes has been discussed many times.
I don’t know Icelandic tax law, but I suspect it requires you, not the customer, to calculate and apply VAT. So your concern about a 1 ISK mismatch with your customer’s purchase order should not be of concern. The customer’s purchase order is not a financial transaction of your company. Nor does it influence your tax obligation to the government.
I also suspect that, like in other countries, your law explicitly allows the line-item calculation method. This is because the government is concerned with getting its due in total, not on the basis of individual invoices. One invoice may round up, the next may round down. In fact, the odds are high that on a single invoice, one line may round up, another down. At any rate, the difference is very small, and as long as the calculation method is legal, Manager is giving you proper results.