sync vs. async exchanges

Use Case to capture:

1. Sofia & Pavlik receive dental health care from Dr. Cesar
2. Sofia requires 3 sessions, Pavlik 2 sessions
3. Pavlik pays to Cesar in cash after first sessions for his both sessions (current and next a week later)
4. Sofia pays to Cesar in cash 10 days after session last session for all 3 sessions

As we see here we have exchanges where physical service gets exchanged for virtual currency. Async aspect works in both directions, one payment ahead of service and one payment after the service.

In both case we have promise that reciprocal Transfer will happen at certain point in the future.