Buyer has bought 2 of the same items in the same sale only wants 1 item how do I cancel just one

[boyals]

Buyer has bought 2 items in the one sale only wanted 1 item how do I cancel just one item when I go to cancell order I have to cancel both 

[kopenhagen5]

If the buyer selected 2 in the one listing then just cancel the order and ask the buyer to re-purchase only selecting one in the listing.