A procedure comprising: a) formulating a proposed instance of a first transaction and sending it to the second party over a side channel, the proposed instance being formulated according to a transaction protocol recognized by the nodes of a blockchain network for validating transactions, and specifying a set of one or more values of a respective one or more parameters of the transaction including at least said amount of the digital asset, b) upon the second party not accepting the proposed instance of the first transaction, receiving back over the side channel a counter-proposed instance of the first transaction, the counter-proposed instance also being formulated according to the transaction protocol, but specifying a modified set of one or more values of the one or more transaction parameters, and c) the first party selecting whether to accept the counter-proposed instance received in b).