Infusion does not have the function to handle Retention's automatically.
Option 1 - Deferred Payments
Invoice out the full amount and receive the balance paid. Enter a credit for the balance and raise a second Invoice for the retention.
This second invoice can then have a Deferred Payment set up (it is recommended to use random dates).
Option 2 - Progress Claims
Set up Progress Claims and invoice as agreed. Leave the Retention until the last invoice.