High risk, nothing but excuses, no guarantees, can't collect, can't evict, not worth the hassle. 

Then, after the sale of this other house which is worth 100k, he would give me the other 40k.

. ($90K is likely resale value.)Second duplex is selling for $80, needs $10K worth of work.

Would it be worth the effort?

If you decide, "hey, I don't want any more than 50% of my net worth to be RE equity" then that will influence your decision.  

So I'd say the seller(s) should consult with an attorney or accountant, but it's possible they may owe much less than 15% or nothing.Also as Account Closed said, seller financing is also often attractive to people who are concerned about taking a big tax hit in one year - instead it's "pay small amounts as you go, over time as you receive the money" and is great for a number of reasons (no or a favorable interest rate for you, no bank BS, build long-term relationships and credibility with future sellers, possibility to discount in future with a payoff offer, etc.).It's worth having in your mental toolbox even if you don't use it on this deal.

For example, you might have something like this:$50,000 - Purchase price for property$30,000 - Rehab cost$100,000 - Your current asking price to sell.There's nothing wrong with that if you can show the property is now worth $100k. 

The house is worth about $95-100k in this market in its current condition.

The cost to even enforce it, if it was a binding agreement, is normally not worth the headache.

Google street view is amazing but since you will Likely only be able to purchase one property in the next 12 months, I would wait until your back in the states.