Since Odoo treats the POS as a full payment you can try these workarounds:

1.  Use Partial Invoicing on Sales Orders (not via POS)

If you need flexible billing (partial payments, installments), it's better to manage it directly in Sales Orders and Invoices, not POS. The sales flow is more controlled there:

• Confirm the Sales Order.
• Create a partial invoice or down payment.

• Manage deliveries separately.

2. Custom Module/ Studio Automation

You can create a custom flow to:

• Prompt for quantity selection when importing a quote into POS.
• Dynamically generate a "partial delivery" POS order.
• Automatically update the original Sales Order to reflect partial fulfillment.

This can be done via:

• Odoo Studio (limited, UI-only)
• Custom Odoo module

3. Create a Down Payment product in POS

If the customer only wants to pay a portion:

• Add a “Down Payment” product manually in POS.
• Enter the partial amount manually.
• On the SO, mark that a payment was made.
• Later, invoice the remaining balance from the SO.

4. Separate the POS Sale from the SO

This is common in some retail/wholesale scenarios:

• Treat POS and Sales Orders as separate channels.
• Use POS for immediate sales only.
• Use Sales Orders + Invoicing for partial/delayed payment flows.