THere are two ways.
1/ The simplest is to generate a new PO and add Charge Items, one for each wrongly costed Item. Just add the additional cost here, and then attache these to the original receipt. You then mark the new Vendor invoice somethign like VI010101-A
2/ You can do an inventory valuation, and use nromal accounting to adjust the vendor aspect.
Once the Item Cost is cahnged, just run Adjust Cost routine and Post to GL,a dn all is fixed.
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