common practice for IP address announcement agreement if addresses belong to other ISP


as probably many of you know, it's possible to create a route object
to RIPE database for an address space which is allocated outside the
RIPE region using the RIPE-NCC-RPSL-MNT maintainer object. For example
an address space is from APNIC or ARIN region and AS is from RIPE
region. However, what if customer, who has it's own AS number, wants
to buy IP transit from RIPE region ISP using addresses from APNIC
region and customer claims that owner of those addresses in APNIC
region allows him to do so? Common sense suggest that if one wants to
announce someone else address space, then they should inform the owner
of the address space and the owner of the address space needs to
agree. Is there a common practice how this agreement should look like?
A simple e-mail? A signed document, which is scanned and sent from
address space owner to ISP who wants to announce those addresses?