The couple were attacked in Majorca on Friday evening with police finding them hours later, on their bathroom floor. Their hands and feet were bound.
Sixty-five year old Rainer Völker was pronounced dead when police arrived at 11pm, while his wife was taken to hospital to be treated for head injuries and shock.
Local daily Diario de Mallorca said the criminals had been searching for the proceeds of a restaurant sale carried out by the couple from Hamburg months earlier.
The couple had apparently sold their restaurant business in Majorca's Cala Millor for €300,000 months to Russian businessmen.
The pair had just sold up and retired, before moving to a quieter area.
Diario de Mallorca said the criminals had believed that more than half of the sale proceeds were still in the couple's home.
The robbers ransacked the couple's home, even taking apart air conditioner units and electrical switches but left jewellery and €1,000 in cash on the premises, the daily reported.
It is not known what loot — if any — they carried away with them.
Police are still investigating the case with police sources saying there were injuries all over Völker's body. The cause of death has not been established.
His 67-year-old wife remains in hospital.
The gangsters are believed to have broken into the flat through a broken garage door, according to German news agency dpa.