Hi!
This is how it works in standard
If you allow change of purchasing price in processing options of P4312 AND the quantity/unit cost is changed as part of the receipt process, then the extended price must be blanked out by the user to allow recalculation.
if you are NOT changing the quantity/unit cost at time of receipt and still get an error, the reason may be a modification in how the purchase prices are calculated on F4311 (perhaps a rounding issue). Turning off the "allow price change" in processing options should remedy this.
Hope this helps