Clear the payer-seat default to Inherit. A Disabled default is a revocation or ACH-return stop and returns 409; replace it explicitly with a new authorized method instead. Requires a session with recent MFA step-up for HighRiskWriteAction. If step-up is missing or expired, this operation returns 403 StepUpMfaRequired; create and verify an MFA challenge at /v3/platform/mfa-challenges with requiredFor: "HighRiskWriteAction", then retry.