Is there such a thing? In my previous working exp. the HR must accept your resignation, and the notice period starts from HR receiving your resignation. That is why, when I tender resignation, I always cc a copy to HR.

As for "buying out" notice, not all companies accept your notice being bought out. This is subjected your current employing company. As for leaves, sometimes, company may not even let you deduct your leaves. they pay you instead. this is normally done if you are stuck in some important project, and cannot be let off.