Error on supplier order with multi-currency

Bug

When we add a product with a buying price in another currency to a supplier order, the price in original currency is wrongly calcultated in most cases.

Dolibarr Version

18.0 and more (at least)

Environment PHP

7.4 and 8.1

Environment Database

MariaDB 11.4.2

Steps to reproduce the behavior and expected behavior

Create a new product from a supplier whose currency is not yours.
Add a price to this product in the supplier's currency with a conversion rate.
Add the product to an order to this supplier, with an important quantity.
Check the unit product in the supplier's currency, and the total amount in the same currency.

On my side, I created a supplier whose currency is Japan Yen with a rate defined at 167. I created a product called Issue with a price with this supplier defined at 23000 Japan Yen (first and second screenshot)

I created a new supplier order in Japan Yen, and added this product with a quantity of 15000.

The result should be the following :

Unit price Excl. vat: 23000/167 = 137,724550898
Total price Excl. vat: 23000*15000/167 = 2065868,263473054
Unit price Excl. vat in original currency: 23000
Total price Excl.vat in original currency: 23000*15000 = 345000000

But, with rounded and current calculation method, we get respectively: 137,72455 2 065 868,25 22 999,99985 344 999 997,75

(Thrid screenshot)

It's a small difference here, but in fact it's due to the original currency price calculation: we start from the unit price in euros here, we multiply it by the conversion rate, and because of rounded values, we don't get the good result. Because of that, supplier orders are wrong, and can't be used to send the order to the related supplier.

I just opened this issue because I wanted to give the possibility to discuss that point, but I'm ALREADY working on that point, so could you please assign myself to this issue while I can't do it on my own?

Dolibarr / dolibarr