But when he opened a door they burst in and struck him on both arms, before ransacking the house and stealing cash.
Det Con David Wheatley, leading the investigation, said: "This sort of crime makes my blood boil."
'Two cowards'
The first man is described as white, about 5ft 6in (1.67m) tall, chubby and dressed smartly in a black jacket.
The other man was white, about 5ft 8in (1.73m) tall, aged between 20 and 30, thin and wearing a black jumper and a bobble hat.
"These two cowards, posing as 'officers', have tricked a vulnerable elderly man and then attacked him with a weapon. How much lower can you get?," Mr Wheatley said.
"We are doing everything we can to find these two thugs and we have increased our patrols in the area, both to provide visible reassurance to any members of the community who are concerned and to try and catch those responsible."
Detectives believe it was an isolated incident but are urging people living in the Osborne Street area to be vigilant and report anyone suspicious.

Read the entire article at

http://news.bbc.co.uk/2/hi/uk_news/england/manchester/8503616.stm