Starting from version 18.104.22.168, you can apply payment policies in reservations.
In Settings => Products => Payment Policies, you can create new payment policies. There are 2 options: you can ask a percentage of the remaining balance, or a fixed amount. You also need to select the pay method.
Then on a reservation, you can automatically apply this payment policy. Under the payments tab, you have a new button ‘Apply payment policy’. This will show all policies with a pay method. Click the one you’d like to use, and it will automatically create the payment.