4 ideas
The Church defined usury as charging for the passage of time on a loan, so the Medici lent through bills of exchange instead. A sum was advanced in one currency and repaid months later in another city at a rate set to favour the banker, which turned the interest into an apparent exchange gain. Theologians tolerated this because the rate might move and the banker bore a real risk, so the profit counted as payment for risk rather than for time.
Each Medici branch (Rome, Venice, Geneva, Bruges, London, Avignon) was set up as its own partnership. The Florentine parent held the majority stake, and the local manager put in a small share of capital in exchange for a share of profits. This structure walled off one branch's losses from the others and gave distant managers a personal stake in results. The cost was that Florence could only control them loosely, through annual accounts, letters and periodic visits.
Tommaso Portinari ran the Medici branch in Bruges. Seeking favour and prestige at the Burgundian court, he lent heavily to Duke Charles the Bold, while Lorenzo de' Medici in Florence paid little attention to the bank. When Charles was killed at the Battle of Nancy in 1477, the debts proved unrecoverable.
Parks reads the family's spending on churches and art partly as a way of cleaning up money made from a trade the Church condemned. Cosimo's large outlay on rebuilding the convent of San Marco reportedly followed a papal suggestion that such gifts would atone for his usurious gains. Seen this way, the Renaissance buildings and paintings are the visible settlement of an unresolved moral tension about finance, not only expressions of taste or power.