Here you can attach put an ad dendum to the existing contract.
Here's how you do this:
1. Click the green "Add" button in the Extra tab.
2. A new contract screen opens where you need to click save to create sales.
3. In the top left corner you can see that a subcontract has been made under the main contract.
4. Now you can add Hospitality / Parking / Sponsoring / Incentives / Installments to this subcontract.
5. As you can see in the upper right corner the Installments have been set up.