How is zakat calculated on money recently returned to you, and is zakat for past years calculated on it, and how is zakat calculated on money not yet returned to you?

If the debtor partner is solvent and willing to pay, then zakat is due on the debt owed by him every year. However, if he is insolvent or procrastinates, then zakat is not due until the debt is collected, and it is paid for one year only.