The 4-building Williams Square office tower project in Las Colinas is among more than a dozen properties across the Sun Belt being offered for sale by HFF. Built in 1985, the buildings are almost fully leased with major tenants including Pioneer Natural Resources, Flowserve Corp., Franklin American Mortgage and Ethos Group Inc. Three years ago when Williams Square sold to commercial real estate investor Brookdale Group, the property went for more than $220 million.