[ggm]

In the Islamic finance model Muslims lending to Muslims, you don't charge interest so much as take a position in an investment with an eye to future value. At least that's how I understand it. There can still be a lein over assets, and a tail of payments and the final state would put both lender and debtor "where they expected to be" under the contract.

Between Christians and Muslims, Christians and Jews, Jews and Muslims different rules may/may-have applied.

Kings (of england, and other places) valued Jews precisely because of the fluidity of their lending to the crown at interest under catholic proscription regarding loans between christians, not the least because at long term consequences in access to jewish loans you could repudiate the debt and (or encourage your citizens to) conduct a pogrom.