How are we to understand the second part of canon 1543?  And, how does the clause play itself out in practice?

Here is the canon in latin -

Can 1543. Si res fungibilis ita alicui detur ut eius fiat et postea tantundem in eodem genere restituatur, nihil lucri, ratione ipsius contractus, percipi potest; sed in praestatione rei fungibilis non est per se illicitum de lucro legali pacisci, nisi constet ipsum esse immoderatum, aut etiam de lucro maiore, si iustus ac proportionatus titulus suffragetur.

Translated into english -

If a fungible thing is given to someone in such a way that it becomes his and later is to be returned only in the same kind, no gain can be received by reason of the contract itself; but in the payment of a fungible thing, it is not in itself illicit to contract for the gain allowed by law, unless it is clear that this is excessive, or even for a greater gain, if a just and adequate title be present. [36]

Google translates praestatione as "performance", where two commentators I read translated it either as "payment" or "lending".  So, if the translation is faulty, take it into account.

My thinking is that the second clause in question refers to currency and commodity fluctuations that businessmen encounter in a world where international trade, multiple currencies, and market manipulation is common.  For example, the rothchilds would move the gold around geographically to strengthen or weaken currencies.  Because, gold inspires "confidence", and wherever it was, a strong currency followed.  And, that affects fungible things(currencies).  But, I want to hear what others think about the clause, because I am not an expert on usury loopholes and papal responses.  

Marlemar - thanks for the recommendation.  It does look like a useful book, and I may read it.