This right complements the rule in section 28 that, in the absence of contrary agreement, payment and delivery are concurrent conditions.

'Unless otherwise agreed, delivery of the goods and payment of the price are concurrent conditions..'

The idiom can be implemented in both single-threaded/serial and concurrent environments, but care must be taken to correctly implement the idiom under concurrent conditions.

The rule that delivery and payment are concurrent conditions ties in with the unpaid seller's lien (see Chapter 12) which entitles him in the absence of contrary agreement to retain the goods until payment.

In this connection s 28 of the SGA should be noted; unless otherwise agreed delivery of the goods and payment of the price are concurrent conditions, and must therefore occur at the same time.