Chapter I. Conditions for Authority of Penalties, Sphere of Application of Penal Law et al.
Art. 8 b
If criminal action is brought in Iceland on account of an offence for which a person has already been subjected to sanctions in another State, the sanctions determined in Iceland shall be correspondingly reduced or, as applicable, cancelled to the extent the sanctions may already have been enforced in that State.
Reduction of national sentence following pre-trial detention - national proceedings
EDIT.