Christian's appearance on TNA's Slammiversary pay-per-view was indeed part of a deal made between the two companies when WWE wanted Ric Flair to appear at the Hall of Fame, according to F4Wonline.com.
Christian was told by WWE to cooperate and do the appearance because it affected WWE getting Flair for the Four Horsemen induction. Christian reportedly didn't want to do the TNA appearance.
WWE gave the approval for TNA to advertise Christian at the pay-per-view on TNA's go-home episode of Impact but Dixie Carter decided against it. Reportedly, Carter wanted to wait and tease the appearance on Twitter as the event got closer to create an internet buzz of speculation.