Cost price change affecting Past Transactions

Good morning,
We are having an issue with all non stock item buying prices.
When we purchase a non stock item the cost of purchase is updated as normal but the new cost impact all transactions fro the same item for the previous months. how is that possible ? aren’t past transaction frozen ?
It appears has an accounting irregularity from our auditors point of view…


New price shouldn’t have any impact on the prices in the old transactions. May I know how do you update item’s cost? Are you referring to item’s Price List rate or valuation rate? Since this issue is related to data, please share much information as possible, so that we can check into it from here.

Good morning,
Thank you for your fast reply.
The item price list is automatically updated when we place the purchase order.
For example, here is one item that we have recently purchased at 13 AED that we used to purchase at 20 AEd last year.
As we invoiced recently (13.03.2017), all transaction of 2016 for this item have been update to a cost price of 13 AED where it was previously showing 20 AED