Thank you @Brucanna and @Tut for your great support here.
This is the example…
On October 18th I bought €2.500 from a forex bureau at 1.010 RWF for €1
On October 25th I paid the supplier in Europe based on a proforma which became a purchase invoice on November 10th when the supplier shipped the goods and issued their invoice:
The purchase order includes the cost of transport that I have divided in three and attributed to three items. Based on what I read in the guides and in the forum, I have not indicated any quantity for these items again, just the total share of the transport which should then go and increase the cost of the goods as a total.
On the same date when I purchased the Euro, October 18th, I added an exchange rate of the RWF to the EUR of 0,000990099.
Once the goods landed here I paid import duties and taxes. I put the taxes for each item together and created a new purchase invoice (where the supplier is customs) as follows:
Agan no quantities, just the total amounts for each item category (not all items paid the same taxes).
Now if I look at my stock of this items, this is what I see:
However, if I take for example item number 1 my actual landed cost should be:
( €110,09 + €100 / 6 ) x 1.010 ) + ( 78.551 : 6 ) = 141.116
but as you can see Manager gives me a unit cost of 126.757 instead.
If I remove the exchange rate added for October 18th and leave only the exchange rate I added for January 1st when the year started (1 RWF = 0,001122 Euro) then the unit cost of the item goes down to 11.298 which is completely wrong.
In all this, there is no VAT indicated anywhere even though we did pay VAT to customs. The problem is that VAT is calculated on the customs’ value of the items (e.g. €110,09 using their own exchange rate of about 990) and on some taxes, but I cannot add VAT to the purchase invoice of the supplier (we do not pay VAT to the supplier) and also VAT must be in RWF not Euro.