I know of a few off of the top of my head. Does it matter whether he is in the future temporarily? I am currently rereading Laying Amongst Dokatsu which would fit with what you are looking for. I am assuming you are 18+, but please let me know if this is not the case so I may post age appropriate fanfiction. Enjoy.
Title: Utter Doom
Author: Tally Mark
Rating: M (NO YIMS!)
Genre: Humor/Romance
When birthday presents go wrong. SessKag Oneshot
www.fanfiction.net/s/4265186/1/Utter-Doom
Title: Impending Doom
Author: Tally Mark
Rating: M (NO YIMS!) This is the sequel to the above fic, but the first one fits with exactly what you are looking for.
Genre: Humor/Romance
When good ideas go wrong. Part 2 of Utter Doom. Sess/Kag Oneshot.
Rated: Fiction M
www.fanfiction.net/s/6366541/1/Impending-Doom
Laying Amongst Dokatsu Reviews
Author: Nostawen Allesiel
Rating: M (NO YIMS!)
They were brought together under unlikely circumstances. A deal was struck, a bargain made, relationships were broken, a new one began to form. And in the center of it all, one human woman managed to alter the course of the past and the future.
Rating: M:? Universe: Canon Status: Incomplete Category: Chapter Stories Created: 05 May 2009 ? Updated: 14 Sep 2009
Genre: Drama, Romance Chapters: 50 Reviews: 70? Words: 383,777 ? Reads: 102,032
dokuga.com/fanfiction/story/1968/1
