I did not realize that 12/31 is the deadline, so I'm scrambling to make sure I have everything done correctly. 

Just make sure to list the timeline you're shooting for, that he needs to meet certain deadlines (i.e framing should be done by this date or a penalty will occur etc) and include the percentage in which you'll be splitting the profits.

They may have a certain deadline to get out of their current place, etc.I would wait until after the close, but it's totally up to whatever you're comfortable doing.

Anyone had success getting xome specifically (or other online  auction house ) to extend deadline?

@Jenifer Levini can probably help you.Monday is a holiday and your specified deadline is Wednesday, so you may want to jump on this proactively. 

And personally, when your money is on the line you tend to push yourself more than a teacher telling you a dead line.

Give yourself a reasonable deadline to purchase something.   6 months, a year, 2 years.   

Every one of those properties had multiple bids on the first day (or by the realtors deadline, ie over the weekend), none of those situations had the realtor ask for final and best.

At the end of the day you are in the right place and still have a couple months to set a strategy before the filing deadline.

And it tends to push things towards the last-minute mad rush to the IRS deadline, cutting more corners and creating more stress for everyone.- This disconnect problem is especially harsh in real estate.