The fact that it was well-established long before op's 1930s movies is attested by this sentence in the transactions of the annual meeting from the south carolina bar association, 1886 and to-day, “free … My company gives out free promotional items with the company name on it. Is this stuff called company swag or schwag?

Saying free or available rather than busy may be considered a more positive enquiry. It may also simply mean that you expect the person to be busy rather than free, rather than the other way round.