The notarial contract of March 9, 1156 preserved in Genoa is the oldest known document with the complete mechanism of trajectory exchange: money delivered in one city, promised payment in another city in a different currency, with implicit interest in the agreed exchange rate. It evolved from the 12th-century notarial deed into the private handwritten letter documented from 1299, becoming the central financial instrument of medieval European commerce and the mechanism the Medici used to evade ecclesiastical usury prohibitions.