contract/contract_payment_mode/readme/USAGE.md

1. Go to Invoicing > Customers > Customer Contracts.
2. Create one.
3. Select a partner to which invoice.
4. If the partner has a payment mode, this payment mode is selected here.
5. If not, or if you want another payment mode, you can change it in the corresponding field.
6. Add a product to invoice.
7. If you create an invoice, new invoice will have the selected payment mode.