Promised future trades as part of legal trades? (Solution described, alternatives/comments requested)
While I used to play with futures trading, I now explicitly prohibit it for the following reasons:


Problematic/ambiguous mechanics
Consider this case: I trade you a brick and a future wood for your wheat.

Later, you attempt to cash in the future trade, but I claim to have no wood.  Unless it's obvious that I'm lying, there's no great way to determine whether that's true or not.  Good sportsmanship will solve this problem, but it's easier if the rules avoid situations that breed mistrust.

Alternately, I have no wood, but do have four sheep that I could trade in.  Can you force me to trade those in for wood to settle the debt I owe you?  (Exacerbated by situations such as, I have ten sheep and a sheep port.)  If I trade the sheep for wood in the process of building something, do you have to be ready to jump in and call in the trade mid-building?

Can I trade away debts, a sort've negative commodity, as I would any other amount of resources?

Are you allowed to try and call in the debt after every card I steal from someone with the robber?  I might have the desired resource, but this can also give you an otherwise unavailable insight into what card I stole.
Onerous bookkeeping
Tracking outstanding debts can be an unpleasant process with no natural mechanism outside of keeping a ledger.  This problem is doubled because for every debt both parties will want to track it to have an accurate assessment of their resources.
Damage to gameplay
The most serious problem with future trades in my experience is that it makes trading too open-ended.  In regular Settlers, there are times when you simply don't have the resources to build everything you want, no matter how much you want to.  You accept it, and continue playing.  With future trades, suddenly there's no limit to what you can offer.  Players go on and on proposing ever more intricate trades so that they can get that great coup this turn.

Veteran players are less susceptible to this than novices, but even they will occasionally fall into gameplay-halting spirals of ever-higher futures offers.