Aareal is working with Societe Generale and Deutsche Bank as its advisers. While it is not launching a formal sale process, it is open to discussing ideas, including potentially selling a majority stake, the people said.

That could value the unit at 500-600 million euros (£432-519 million), they added.

"Now is a good time for potential buyers to get in touch with Aareal", one of the people said, adding pressure from activist investor Teleios Capital Partners had changed the dynamics of the situation.

The banks and Teleios declined to comment.

For Aareal, finding the right investor with expertise in software and property is more important than maximising price, the sources said.

An earlier attempt to sell a 30% stake has so far not borne fruit, but selling a minority holding remains Aareal's preferred option, one of the people said.

In a recent letter to Aareal, Teleios said an M&A adviser with expertise in large software deals would be best placed to manage a sale process.

The investor has threatened to call for an extraordinary shareholder meeting and a special audit if its demands are not met.

Aareal's chief executive officer Hermann Merkens earlier this month wrote in a letter to Teleios that Aareon was a "key pillar" and that the bank was continuing to grow and invest in the division.

But Merkens also said the group was "assessing all relevant options" and had no established preferences.

Teleios, which holds a 4.5% stake in Aareal, has said investors may be better served by a full sale of Aareon, adding the unit could be valued at 700 million to 1 billion euros.

Aareon's software helps professional landlords manage their properties and rental payments and lets customers to park rents with Aareal, giving the bank an inexpensive way to refinance itself.

(Additional reporting by Tom Sims; Editing by Tassilo Hummel and Mark Potter)

By Arno Schuetze and Patricia Uhlig