Generally you would pass this information off-chain, e.g. via an email message or web API that you developed yourself. However you *could* embed it in the metadata of another transaction that it sent to the recipient, with asset value zero. Just do something like this:
sendwithmetadatafrom [sender-address] [recipient-address] 0 [full hex blob]
Then the recipient would receive this transaction and could retrieve the exchange offer from the data field of the response to listaddresstransactions. However depending on your use cases this method may have some disadvantages, because:
- Every node on the blockchain sees this offer.
- Any other node on the blockchain can complete the offer.
- The offer takes up space on the blockchain.
MultiChain does not yet have an integrated layer for off-chain messaging, though it's something we have certainly thought about as a future feature.