Allocates a transaction to a plan (reconciliation). By default the full remaining unreconciled amount is applied; pass amount to allocate part of it. To split-fund a plan that arrived as several credits (e.g. a $3,800 invoice as $3,650 + $150), call this endpoint once per transaction with its portion. Use is_payment=false to link a transaction as an internal transfer without creating a payment or changing the plan's collected amount. Returns the updated transaction with its reconciliation summary, allocations and payment_ids.