You can now setup a future instalment without specifying a date to this charge - the charge will be based on a Milestone instead.
For example: You could accept an amount upfront, and then charge the rest of the payment upon acceptance into a program.
Add your single payment Price
Click on Go advanced.
Add installment, On a milestone:

Enter the Milestone and Instalment amount:

A FEW THINGS TO NOTE!
You will need to MANUALLY collect the instalment on the purchase when the customer reaches the Milestone you set. The system will not be able to do this automatically.

A Milestone instalment can only be followed by another Milestone instalment!