Standard Life is understood to be buying Number One First Street from German fund Patrizia Immobilien AG for £70m, a net initial yield of 5.5%.
Patrizia bought the 180,000 sq ft office as part of the wider 20-acre First Street development area from Ask Real Estate in May last year.
The block is fully let, to tenants which include Ford Credit, Auto Trader, and Jacobs, with a Pizza Express restaurant in the ground floor.
At the time of the purchase of First Street Patrizia said that its focus was on new developments at the site, with plans to bring forward 1m sq ft of offices, alongside a private rented sector residential scheme.
Patrizia is currently working on plans for 624 PRS apartments on a three-acre plot at the southern end of First Street, designed by Callison RTKL.
Earlier this year Patrizia announced that it had sold the Melia Innside Hotel to a UK hotel group for around £30m, with the money being used to invest in the PRS element.
The German fund is also working in a joint venture with the Greater Manchester Property Venture Fund to bring forward a £75m office next to Number One First Street, which has been partially pre-let to Gazprom.
JLL is advising Patrizia.
All parties declined or were unavailable to comment.