If the distribution is incorrect when matching an invoice to a PO, the changes should be made at the invoice processing stage

If you feel the PO distributions require altering then:
•    Within AP, match the PO to the invoice as normal (with the incorrect distributions).
•    Enter two new lines: the first a negative line to remove the value from the incorrect distribution and the second a positive line to the correct distribution code. 
•    Be careful to remove the PO number that defaulted in on both your new lines as you do not want to unmatch the PO. 

This will leave the PO status as 'Matched' and the GL entries correct. If required, refer to guidance on matching an invoice [1].

The system tolerance levels, used when comparing the purchase order line amounts and invoice amounts, have been changed in AP to allow invoices with a 5% variance (up to a maximum of £100) to proceed without creating a hold.