Open POS - New Currency Frontiers

This is the generic situation of purchasing products or services in a store or on-line. We have a customer (C) with an intention (I) that might be satisfied by one or more products (P) available from a vendor (V). C shops and identifies P to satisfy I, possible with the help of V. Now we are read for the POS dance.

Create a purchase document for the sale stating the terms of exchange. The document must say who (C) is purchasing which (P(n)) from whom (V) and the price, what currencies at what rates.
Customer agrees to the terms and signs the document (digitally)
Vendor accepts terms and signs. This can be in either order, and the result must be a purchase document with signatures of V and C.
Record the transaction. (each party can record it separately, and one or more servers can provide a signed receipt, a copy of the signed document, signed by the server)
Vendor and Customer can now independently produce evidence of the exact transaction terms including the agreement of both parties.
Good or services are delivered.