Specific SQL For Total Billing

My DB has the following description

Table & Columns: ID field is identity. Currency is either CDN or USD.
Order: ID, DateOrder, Currency, AmountOrder
Invoice: ID, NumOrder, DateInvoice, AmountInvoice
Credit: ID, NumOrder, NumInvoice, DateCredit, AmountCredit
Rate: YYYY, MM, US


Relationship among tables:
Order & Invoice : on Order.ID = Invoice.NumOrder, one to many
Order & Credit: on Order.ID = Credit.NumOrder, one to many
Invoice & Credit: on Invoice.ID = Credit.NumInvoice, one to many

Other conditions:
For orders of currency USD, the rate of the DateOrder is used to convert the Order.AmountOrder to CDN.
The rate of DateInvoice is used to convert the Invoice.AmountInvoice to CDN.
A credit always uses the rate of DateInvoice of its associated invoice.

Report: Total billings of USD sales in a period of time (startDate, endDate), converted to CDN

Total billing of USD sales converted to CDN = sum [AmountInvoice * rate of DateInvoice – sum(AmountCredit * rate of DateInvoice)]
WHERE the DateInvoice and DateCredit must be in the specified period.
AND associated Order.Devise = USD


SQL:
1 SELECT SUM (AmountInvoice*dbo.GetRate(DateInvoice) – SUM (AmountCredit*dbo.GetRate(DateInvoice)) as TotalBilling
2 FROM Order IINNER JOIN Invoice ON Order.ID = Invoice.NumOrder
3 LEFT JOIN Credit ON Invoice.ID = Credit.NumInvoice
4 WHERE Order.Devise = ‘USD’
5 AND DateInvoice >= startDate AND DateInvoice <= endDate
6 AND DateCredit >= startDate AND DateCredit <= endDate

Actual Results: Only orders which have both invoices & credits. The orders which invoices but no credit are not included in the results.


How can I correct this, please?