The 55-times capped Harlequins centre, who was awarded an MBE for services to rugby following the World Cup win of 2003, announced his retirement from the game in March.
Scotland head coach Frank Hadden will use the visit of the Baa-Baas as the start of his preparations for the 2007 Rugby World Cup ahead of the two-Test series in South Africa next month.
And the 33-year-old recognises the task ahead for the invitational team, who lost to Scotland for the first time last year in Aberdeen.
Greenwood said: "I've been through this already having played my last game for my club side, Harlequins.
"But this is an extra opportunity to play for a team I've only played for once before and have a bit of fun with a few mates.
"At the same time, we understand the size of the task against Scotland.
"It is not all about beers and golf - we are focused on our task at Murrayfield.
"Scotland have been doing extremely well over the past 12 to 18 months under Frank Hadden.
"We applaud what they have achieved and are under no illusions over to the task ahead of us.
"But, at the same time, you put that shirt on along with your own club socks and try to express yourself.
"We may run out of steam but we'll have a go and see how we get on."
Greenwood insists it is a "great honour" to end his career with the Barbarians, but he is determined to go out on a high.
He added: "I've been to Murrayfield a couple of times and enjoyed some half-decent results. But it is always a tough game, whatever the scoreline.
"The Scots have a great pride in their national shirt and understand what they're representing.
"They are preparing for a very tough tour of South Africa and will want to get their patterns in place and some good rhythm and record a good win to go down there in great confidence.
"We are an obstacle in the way of that and we will try and do our best to disrupt that.
"I might need a lung replacement after about 45 minutes, but we'll see how we get on."