or contracts from places like legal zoom good enough?

Do not rely on it as legal advice.

Essentially Partner B is building all of the sweat-equity into the property.So the question is, after both parties have been paid back their initial capital investment, in your opinion how should the ownership of the property be structured on a percentile basis?

He knows how it works, and he's creative enough to find ways to legally do things.

I assume a simple short message on legal pad yellow paper in an envelope would do it, but what about postage?

I'm late to the party but your original post raises red flags to me.

But I have never seen a property sold to a third party, then un-sold to reopen bidding. 

Hope that is still legal.

What's the terms of your agreement; how many days do you have to assign your contract, what happens if you don't find an assignee, do you have a legal and air tight assignment agreement, are you legal (not acting as an agent), is there real consideration, how much and who is holding it consideration, do you have a built in extension in the event you don't close in time, do you have right of possession durning contract period, what showing arrangement do you have?

I recently learned that I can't legally charge a management fee in state of Oregon for "managing" a small residential property.