He mentioned our soul but I'm not exactly sure what does it mean in the FR settings
Your companions also suggest you in not dubious terms that "asking for your soul" is probably a red herring and he's actually interested in your tadpole, specifically.
I'm still convinced that these "special tadpole" are somewhat tied to some some big conspiracy involving some other god resurrection or something of that sort.
By the way, has anyone tried to agree to a deal with him? What happens in this case?
I didn't try, but I read some comments about it. Apparently he still send you back to your camp with the vague promise you'll be seeing each other in the near future.