ما هي حكمة تحريم ربا الفضل؟ وما هي أشكاله المعاصرة؟

The prohibition of riba al-fadl (usury of surplus) is due to its being a means and a pretext for riba al-nasiah (usury of delay). Riba al-nasiah is the primary type of usury intended [for prohibition]. Riba al-fadl was forbidden as a prohibition of means and a blocking of pretexts. An example of this is exchanging old gold for new gold with a difference in weight.