A criminal investigation is under way into the payments made to the Geneva lawyer between 2007 and the beginning of the summer of 2015, broadcaster RTS reported on Monday.
The unidentified lawyer and the unnamed senior civil servant at the cantonal hospital have been charged with collusion and abuse of public trust through management, the state broadcaster said.
They were detained before being released last week while the investigation continues, according to the Tribune de Genève newspaper.
Officials at HUG reportedly became aware of the massive payments just a few months ago.
The lawyer was apparently retained to recover payments from the hospital's debtors.
The investigations aims to determine why these fees were paid because the pay orders were not formalized in written documents, RTS said.
The investigation aims to establish responsibilities for the hefty payments, including at the top of the hierarchy of HUG.
The cantonal hospital and Mauro Poggia, the Geneva cabinet minister responsible for health care, have declined to comment on the affair.