Royal Bank of Scotland sold its downtown Stamford office building for $163 million this week to Philadelphia real estate-investment firm Rubenstein Partners, a deal that marks the city’s largest property sale this year. Rubenstein’s acquisition, which netted the state about $2 million in conveyance taxes, will complement its co-ownership of the Shippan Landing complex in the city’s Shippan section. Commercial real estate firm JLL, RBS’ exclusive U.S. leasing and sales agent, brokered the sale.