I think that this is the same thing as contractum trinius, a legal maneuver used by European merchants to avoid anti-usury laws in the medieval ages. It makes very little sense reword in the problem in terms of puts and calls because according to Islam options are a form of gambling. In it's infinite "wisdom," Islam not only fixes the risk-free rate at 0%, it also prohibits financial risk-taking (gambling). This really isn't anything nearly as complicated as arbitrage or option pricing, this is just a series of very simple cash transactions which (when combined) replicate a fixed-rate loan. The only amazing thing here is that it was only in the late 20th century that Muslims finally discovered this loop-hole, while the Jews and Christians had been doing it for centuries.