I need to allocate IPv6 /48 block from one provider to another, located in different country. Is it possible at all? I don't own this block but let's assume that current provider will agree to "delegate" (don't know if it's proper term) it. What steps should be taken to make this work? Thank you in advance for any suggestions.
(The word is "announce", not allocate.)
You have Provider Assigned ("PA") address space. It may or may not be "portable". In order for you to "dual home" that prefix, the owner of the address space would have to give you written permission to announce it outside their network. Everyone involved would have to announce that specific (/48) prefix. (A /48 is "more specific" than a /32 (provider block))
I don't know about your ISPs, but the one's I'm familiar with in the US really don't like doing that. And as IPv6 "PI" (independent) space is easy to obtain, there's no reason for them to allow such unproductive dilution of their address space.
If the provider agrees then you would need an ASN besides their written permission and BGP setup in your local / destination site.
Setup a router and announce that address space to upstream with your ASN.
I hope this will help.