There have been many studies by the timber framing guild engineering council on scarf joints. You should research this idea of a scarf joint over a post. It is usually recommended to NOT be over a post. When you put a timber over a post you are asking that timber to give you all it can give you to support your frame. Then you can half of it away. How can a timber give it all the strength you need if you cut half of it away? Do more research on scarf joint locations and find a better location for your scarf joint.