Derek is businessman, stiles is writer, established relationship and misunderstandings.
It was on ao3.
Derek and stiles are together in the beginning Derek works somewhere and is at a business dinner which Stiles apparently promised he'd be at but didn't. It turns out he was at a surprise birthday party his friends threw him that he thought Derek would be at.
They're getting in an argument about it when their neighbor interrupts when Derek comes back Stiles is gone and left his key and that's their break up.
It's a happy ending only 1-3 chapters long and complete.
*Found! It's You Make My World Go Round (Sorry) by Heather_Night