Think about the type of deposit too. I wouldnâ€™t put it past someone to transfer $1MM in a 3 month cd, get the rate discount for 30 years and transfer the funds at maturity, especially if your rates were competitive before the discount.
Banks have used established customers, and deposit balances to reward customers. The amount seems very high though. Since minorities generally have a greater chance to fall into the lower income/asset ranges, you need to document (as the others said) the reasons to avoid disparate impact.
Integrity. With it, nothing else matters. Without it, nothing else matters.