Original PR description

Version- 17.0

### Issue:
- When an upsell order is partially paid (e.g., 10% of the original amount), the generated downpayment invoice incorrectly displayed an invoiced amount of 0.

### Steps to reproduce:
- Make an Upsell order of subscription.
- Update the condition of Online Payment from 100% to 10% (in this case).
- Make the payment of 10% from portal view.
- The invoice amount will be shown as $0.

### Fix:
- Updated `_get_subscription_qty_to_invoice` to correctly determine the invoiced quantity and amount for downpayment invoices on upsell orders.

### Impact:
- The downpayment invoices for upsell orders now correctly reflect the actual invoiced amount.

Original PR description

**Issue:**

When creating a vendor bill or vendor refund through the Documents module after selecting a supplier, the currency defaults to the company's currency instead of the vendor's.

However, if the supplier is selected later in the Accounting module, the correct supplier currency is applied.

**Steps to reproduce:**

  - In Documents, upload a bill.
  - Click on the bill and assign a vendor (whose supplier currency is different from the company's currency).
  - Click on "Create Vendor Bill".

The used currency isn't that of the supplier.

opw-4406074