複数の結合を使用すると、Mysql合計クエリが誤った結果を返す

結合によって必要な組み合わせでより多くの行が作成される可能性があるため、合計を直接使用してください。

次のことを試してください：

SELECT id, MAX(Total) as FinalTotal ,MAX(Payment) as FinalPayment
FROM si_invoices a 
    left join 
    (select invoice_id, sum(total) as Total from si_invoice_items group by invoice_id) b 
    on a.id = b.invoice_id
    left join
    (select ac_inv_id, sum(payment) as Payment from si_payment group by ac_inv_id) c 
    on c.ac_inv_id = a.id 
group by id

またはidが一意の場合：

    SELECT *
FROM si_invoices a 
    left join 
    (select invoice_id, sum(total) as Total from si_invoice_items group by invoice_id) b 
    on a.id = b.invoice_id
    left join
    (select ac_inv_id, sum(payment) as Payment from si_payment group by ac_inv_id) c 
    on c.ac_inv_id = a.id