If you see a negative sale (or debit) for iTunes in your Monthly Sales Report, that means someone purchased your album through Complete My Album (CMA). You can learn more about Complete My Album below.
iTunes lets customers who purchased one or some of the tracks from an album complete the album purchase without having to pay the full album price ("Complete My Album" or “CMA”). The iTunes customer pays an adjusted price to obtain the additional tracks in the album. For accounting purposes, the CMA album will be accounted for as a sale of a full album, and the previously purchased track is considered a return (this will show in your monthly sales report as a negative sale).
If the purchaser buys a song (i.e. as a single before the album is available), the purchaser can only buy the album, when available, as a CMA purchase if
- The song on the album has the same exact track title, artist name and track length as the single;
- The same ISRC is used on the song;
- The album price is equal to the sum of tracks.
There will be a 1-week sales reporting delay for your monthly sales report each time a CMA purchase is made.
If you had an album in iTunes U.S. with 15 songs and one of your fans purchased one song for $0.99 from the album(say, track #8) you'd get the track Pay Rate of $0.70. If the same customer came back and later purchased the album in its entirety as a CMA, you would earn the Pay Rate for a whole album sale for albums over 10 songs/tracks: $7.00. A refund for the original track #8 purchase would be generated. It would "cancel out" the next sale you had of that particular track.
That is, if in the same month, someone purchased track #8, that sale would be "cancelled out" by the CMA refund and would not appear on your sales report. The refund remains until cancelled out by a sale, and can persist over many months until another track #8 is purchased.
If the same version of a song (the same exact recording with the same exact ISRC) is available for purchase as a single and it also appears on a full length album, a "Complete my Album" option may show on the full length album for iTunes customers who already purchased the single. The "Complete my Album" option will not appear if the same version of a song is available for purchase as a single and on an EP (6 songs or fewer).
You can learn more about iTunes' Complete My Album feature here.