The reason why I had Ben buy the tree was to keep the example simple-- ie, it used only two protagonists.  Obviously in real life there are maybe many other people involved, eg: Ben would loan the money from a bank as you pointed out.  However, it is conceivable that such a situation as I outlined could occur-- eg: Ben bought the trees because Satoshi didn't need it all (all the wood was really worth 70BTC whereas Satoshi only wanted 55BTC worth), so Ben kept some himself for his own cooking, warmth, to build a hut and so that he could sell some to other people.  In fact I know of a situation where something like this happened in real life.  A while ago in my country some of the banks started selling of their physical buildings to investors with the condition that they be rented back to them, the bank even supplied a loan for you to buy them if you didn't have the cash.

Regardless of this though, the point I was trying to make is that the claim that it is impossible to pay off interest in a world that is based on a finite money supply is wrong.  This example clearly disproves this claim.  By-the-way, it is not the only way to disprove it, just one of the simplest examples.  Another example that is easy to describe is one where the person who owes the interest pays it back with labour or goods instead of money.  I didn't use this because I wanted to show that you could destroy the interest with "money" alone (where money includes debt in it definition).