Scanned Doc - selecting Item overwrites Rate

Scanned Doc - selecting Item overwrites Rate

I have a Vendor Invoice which was uploaded to Documents. 
I select Add To > New Bill.  
The OCR is actually quite good, but it is reading an Item Description instead of an Item Number.  I remove the description and select the correct Item Number... and it overwrites the vendor supplied Rate with the Rate defined on the item!  On the Item, I have to select it as Purchaseable... which requires me to enter a Pruchase Rate.  Using 0 just overwrites the OCR'ed value with 0.  
In this world of tariffs and price increases, the rate on the Vendor Invoice is usually correct.  

How can I have it not overwrite the OCR'ed rate?  

    Nederlandse Hulpbronnen