Optional. Omit to use the fully-computed invoice schedule. When provided, it replaces the FIXED billing layout: submit any set of invoices you choose (split, merge, add, or drop). Rules: for each contract item, the sum of its submitted FIXED line totals must equal its computed contract value (redistribution only); every FIXED line must carry its contract_item_id; invoices must have distinct issue_dates, each within the contract period and outside any closed book-close period; for FIXED lines total_amount must equal unit_amount * quantity, and amounts on discounted lines cannot be changed (they may be regrouped, not re-sliced). USAGE and USAGE_COMMITMENT billing is computed by the system and must not be submitted; it is kept unchanged. Revenue recognition and TCV are unaffected. See ContractInvoiceScheduleItem for the per-line rules.
