Am I religiously obligated to pay blood money (Diyya) because my failure to provide a medication, which was unavailable, led to a patient's death?

If the doctor who ordered the medication was in charge, then it was obligatory to carry out his order. You erred by not bringing it based on your colleague's words, and you bear responsibility before your employer. As for the man's death, you are not considered to have caused it, and neither diyah (blood money) nor kaffarah (expiation) is incumbent upon you.