The fact that they're offering to rent you a house sight unseen AND that they're in Africa are very strong indicators that it's a scam. Usually, when a person needs to rent their house but is out of the country, they leave the keys with a realtor or a friend who can do showings. It is an EXTREMELY bad idea to rent a property sight unseen, and one of the reasons is because of the high probability that it's a scam (also, the place could be completely wrecked or unsafe inside). The fact that they're asking for money via Western Union is also a red flag- you're completely unprotected if anything goes wrong.
Check to see if the person who sent you the agreement is a member in good standing of the Oregon Bar. If you find someone by that name, call the number on the website to ensure that it's the same person.
You can also visit the website for the County Assessor in this county to see who actually owns the property in question. Not all counties will give you the owner's name, but some will. Then, use Google images to search for the pictures of the inside of the property and see if they come up on other websites (especially if the other sites have a Realtor attached to them or give a different owner's name). You could even knock on the door, tell the people living there your situation, and see if they can tell you (a) whether they're even moving and (b) who the owner really is.
But based on what you've said, this sounds very much like a common scam.
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