Spike gives Gem of Amara to Buffy.

If I remember right, Spike got the Gem of Amara, and instead of going to fight Buffy, he takes a stroll in the sun for the first time in 100 years. I think Buffy or one of the Scoobies still sees him and immediately research and find about the Gem. They find him on a beach staring at the sea or something and Buffy goes to him and gets him to give her the ring temporarily or something like that. I think they came to an agreement where he could use it, but only during brief moments of the day or something. It's been a while since I last read it.

EddyxMarvoloxRiddle
This is 'Better Things To Do' By BeneficialAddiction. They also have up a continuation of the story as well. It's called 'Better Things To Do (A Continuation)'

It's also all under one continuous story (40 chappies) On Elysian Fields - Better Things To Do