{Seward was Secretary of State and as the head of the United States Department of State, his duties were concerned with foreign affairs. As Secretary of State Seward was the highest-ranking cabinet secretary both in line of succession and order of precedence. Seward was negotiating with South Carolina, looking for compromise, and a solution. His verbal promise may not have been legislative law or binding contract, however, it is part of the process, and part of that process is full faith in the word of a government official}
Only if Condoned by His boss, or Congress. Which it wasn't, as he was going behind everyone in the Cabinet, and the President. And, since Lincoln didn't recognize the Confederacy as a foreign country in the first place, then Seward was completely out of bounds and acting illegally. Since Davis had been a former Sec. of War, he was surely aware of this.