Spuffy get sucked into a portal. Only demons can come out. The only way for humans to leave is if a demon gets a soul and links themselves with the human. Time moves differently in the portal dimension. For Buffy it's hours, for Spike it's weeks to months before he can get to Africa and get his soul. Anya helps him get the ingredients for the spell.

Hi,

Could it be Melting Fire by Freecat?

Summary: Set right after Dead Things.
The night after, all he wants is talk.
The night after, there's nothing she wants less than talking.
And suddenly they find themselves in another dimension; one that Buffy can't leave. There's only one way to get her out. A way with consequences.

Hannora
That was it. Thank you!