BULGARIA - PENAL PROCEDURE CODE
Part two. EVIDENCING
Chapter thirty six. PROCEDURES IN CONNECTION WITH INTERNATIONAL COOPERATION ON PENAL CASES
Section III. International Legal Assistance in Penal Cases
Art. 473 (2) Turning over persons detained in custody, in order to be interrogated as witnesses or experts, shall only be allowed in exceptional cases at the discretion of the respective district court on the grounds of papers submitted by the other state or the international court of justice, provided that the person gives his/her consent for being turned over and provided that his/her stay in the other state will not extend the term of his/her detention in custody .
EDIT.