Hi Susan,
What setting do you have in the PO company profile, on the Integration tab. for 'Update base price on entry'? Is it 'No', 'Manual' or 'Auto'?
If you have either Manual or Auto as the setting, then posting Receipts will update the IC master item base price with the value displayed in the receipt detail.
But I could not duplicate a problem when posting the invoice/adjustment and having the IC Base Price change, even with the Auto setting enabled. We'd need a little more information as to your setup and also if anything was changed on the inv/adj detail.
Of course, if you don't want PO to update IC base prices, you can turn off that option in the company profile.
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Regards,
Softrak Tech Support