Maybe this?
http://www.mangago.me/read-manga/akunin_o_nakaseru_houhou/
Not a oneshot though.....
No this isnt it, but thanks for your response anyways!
This one maybe?
http://www.mangago.me/read-manga/otonage_karikawa_seyu/
Its not this one either. I wish I could remember more about it! Its so frustrating! Thanks so much for your response though, this manga looks interesting also!
"secretary" from Minase Masara
http://www.mangago.me/read-manga/secretary/
wow its a good one-shot
And he's having an affair with the Uke, who I think works in the same office as him. The seme seemed older and well dressed... that's all I remember about him. I think it was a oneshot. In the end, the uke realizes he actually loves the seme, but thinks that the seme wants to break it off with him, and stops seeing him. Although, really the seme was just working to get a divorce so he can be with the uke.
I've looked everywhere for this yaoi, but I can't seem to find it!