Merchant settlement and payout batching

Design the merchant settlement/payout system for a payments platform serving 500K merchants. Each day it must net each merchant's captured payments minus refunds, fees, chargebacks, and reserves, then initiate ACH/SEPA payouts on the merchant's schedule (daily, weekly, on-demand). Payouts go out in bank cutoff windows. Describe how a payout amount is computed correctly and immutably, how you guarantee a settlement batch is sent to the bank exactly once, and how you handle a payout that the bank later returns/fails.