This is one of the common ways scammers operate on freelancer platforms like Upwork. He does not want the Upwork platform to be the intermediary and guarantor between you and him but aims to lure you off the platform to scam you and steal your deposits made in this new suspicious exchange.
I hope you did not follow what he asked of you and avoided this unknown Kaspachain exchange, which has no reviews in the search engine or sufficient information about who runs it. Now, you only need to open this person’s profile on Upwork and click "Report this user." I also recommend that you read this article [
How to Identify and Report Scams on Upwork], where you will find many important tips to avoid falling victim to scams, including not communicating outside of Upwork or accepting payments outside the platform.