Processing client-side business rules - the dilemma

I have a Purchase Order to Payment scenario.  Each PO can have many payments, which decrement it's initial amount.

When creating the Payment, I want data from the PO to auto-populate and I need business rules to run on that data; i.e., if the date of the payment is outside the range of the dates on the PO, it's invalid. 

I have two options:

1. Use Entity Attribute Mapping to push data from POSO to the Payment.  This only works, though, if the Payment is created directly from the PO.  If the user clicks the NEW button from a View or a Form, then this solution breaks.
2. Use real-time workflow, but this requires I save the Payment before seeing the values.  I don't understand "real-time", I guess.  

I'm seeking ideas, clarifications from the community.  And can someone explain what "real-time" workflow means if I have to Save the record before it works?