I haven't read all the indictments and so might be wrong, but at this point I think it would be speculation. Time will almost certainly tell for sure. The wikipedia definition of the term seems easy to agree on — https://en.wikipedia.org/wiki/Asset_(intelligence)
Manafort's actions in Ukraine paint him as a Russian asset in my mind (http://time.com/5003623/paul-manafort-mueller-indictment-ukr...). You're correct that this doesn't prove he was acting on behalf of Russia when he offered to work on Trump's campaign for free though. That is speculation on my part, but seems fairly likely.