If the wording in RIPE stated the word "lease", why wouldn't that work? A customer comes and "leases" IP space from RIPE for the duration of their contract with RIPE (membership dues, fulfillment of rules, etc.) When their membership ends, the lease is broken and the IP space returns to RIPE.
People are talking about direct LIR to LIR transfers where the IPv4 space does not return to RIPE.
I bring your attention to: http://www.ripe.net/rs/news/global-ipv6-assign-2001-12-22.html
4.1. Address space not to be considered property
The transfer policies under discussion are for IPv4 only, not IPv6. By the way, I agree that the best way to handle IPv4 transfers is for LIRs to return surplus address space to RIPE and for RIPE to allocate/assign those blocks in the normal way. The only real change needed is to add some kind of first-come first-served rule in case the demand for IPv4 is higher than the supply. --Michael Dillon