IBM mainframe database and communications software development and marketing company Software AG of Darmstadt, West Germany is making an unsolicited tender offer of $11.50 per share for the 78% of its US partner it does not already own. Value of the offer for Software AG Systems Inc of Reston, Virginia is put at about $50m. Software AG Systems looks an attractive target because it has about $20m in cash and no debt. It reported net profit of $4.3m on turnover of $67m for 1987. It says that it will consider the offer on Friday and give a response Monday. Software AG Systems Inc was formed 15 years ago and conducts joint development exercises with the West German company.