If you get stuck on any part I could do more screen shots.
There are 3 steps
1/ Make a PO and put it on order
This will prompt you with the current cost price form the product record (unless you have a supplier price list) as the default price for the item but you can override this.
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I will overrode the price from 1.60 to 1.50
so the product record still says 1.60
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2/ Receive delivery of the goods into stock (or amend if receiving part of the delivery
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These will go into stock at the price on the PO 1.50
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And update the last cost price on the product record.
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3/ Update the ledgers from PO this is the invoice entry.
This puts the invoice on the supplier ledger.
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Press update if everything is correct, or edit if you need to change something, any change made to the unit price will not alter the price the item went into stock at as that happened when the good were delivered and can not be changed now. (Except by manually adjusting stock out and then in again)
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Hit the save button and/or change any of the values needed.