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.