Like today, coins were not the only ways of paying. Checks were around centuries ago as well. "Check" comes from the Arabic sakk, a written vow to honor payment for merchandise when its destination is reached. In the time of Harun al-Rashid in the ninth century, under a highly developed banking system, a Muslim businessman could cash a check in Canton, China, drawn on his bank account in Baghdad. The use of sakk was born out of the need to avoid having to transport coin as legal tender due to the dangers and difficulties this represented. Bankers took to the use of bills of exchange, letters of credit, and promissory notes, often drawn up to be, in effect, checks. In promoting the concept of the bill of exchange, sakk, or check, Muslims made the financing of commerce and intercontinental trade possible.
I don´t know if the monetary system was debt and usury-free. If it was, it might help answer the old question of how a trade-intensive economy can function without these scourges.
A curious aside (p. 151):
Twelve hundred years ago, while the caliphs ruled the Muslim world, King Offa reigned in England. He introduced silver coins to his kingdom and also minted a gold coin, the gold Mancus, worth 30 silver coins. The extraordinary thing about the Mancus was that it copied the Muslim gold dinar of the Abbasid Caliph Al-Mansur, dated 774 AD (157 AH), carrying on one side the Arabic inscription "There is no Deity but Allah, The One, Without Equal, and Mohammad is the Apostle of Allah." A significant difference from the original dinar is that King Offa stamped his name on it with the inscription of OFFA REX. Scholars have puzzled over why an English king would have made a replica Arab coin. Some say he had converted to Islam, but the more likely explanation is that it was produced for trade, or for pilgrims to use as they traveled through Arab lands. The coin most certainly would not have been made by an Arab craftsman because there is no understanding of the Arabic text: "OFFA REX" is upside down in relation to the Arabic Kufic script, and the word "year" is misspelled in Arabic. The coin was probably copied by Anglo-Saxon craftsmen. Discoveries like this have helped us to redraw the international economic and trade relations of 1,200 years ago. King Offa's coin is evidence of how far Islamic currency had traveled by the eighth century. Archaeologists have found thousands of Muslim coins in modern-day Germany, Finland, and Scandinavia, showing that this currency was transported and traded from Muslim countries across Europe. King Offa was not the only non-Muslim ruler to make an Arabic coin. An 11th-century Spanish Catholic prince, Alfonso VIII, ordered the minting of a decorative coin on which the inscriptions were written in Arabic and on which he referred to himself as the "Ameer of the Catholics" and the pope in Rome as the "Imam of the Church of Christ."
I recommend the book, even if it seems to advertise a bit too enthusiastically for the Islamic genius. The "1001 inventions" described belong mostly to the golden age of Islamic civilization (9th - 13th century). In any case, the facts can (and should) be checked in other sources.