Update payer-global settings for the effective Account. Contractor and W-2 payroll resolve the same default or currency-specific debit source. Payable terms resolve engagement, relationship, then effective payer Account defaults; payee-issued Invoice terms remain payee-controlled. Auto-pay for both document types resolves engagement, relationship, then effective payer Account defaults; otherwise auto-pay is off. Changing a default reaches relationships and engagements without an applicable own value; explicit values remain unchanged. Null removes an individual stored default; omitted fields stay unchanged. defaultPaymentMethod updates the canonical Client profile pointer; InternalAccount is currently rejected with 422 payments.PayerSettingsDefaultPaymentMethodUnsupported. Person sessions, API keys, and ServiceAccounts are accepted subject to the PayoutMethodChange elevation policy.

Requires a session with recent MFA step-up for PayoutMethodChange. 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: "PayoutMethodChange", then retry.