The county is conducting final inspections on the Royersford facility and hopes to complete the $39 million sale to Mid-Atlantic Health Care in the coming weeks.
Greenleaf, a Republican row officer in charge of county payroll and accounts, reviews all county contracts and audits county departments.
Commissioners Chairman Josh Shapiro said Friday afternoon that he did not know the reason for the subpoena.
At the meeting, Commissioner Bruce L. Castor Jr. pressed Greenleaf on whether the commissioners should proceed with the sale.
"I don't think it's my place to counsel you on that," Greenleaf said.
"But given that the county is engaging in a transaction that is tens of millions of dollars at play, as a public official of Montgomery County, would you . . . let us know if we're about to make a $40 million mistake?" Castor asked.
"Of course," Greenleaf answered.
Greenleaf would not identify the employee but said it was not Parkhouse administrator Melanie McGarry, who has supported the sale.
Greenleaf also would not say why he chose a subpoena rather than merely asking the employee to comply with his investigation.
The commissioners began studying a sale last February and voted unanimously in October to accept Mid-Atlantic's bid, arguing that the facility would be better run in private hands.
A group of residents has been protesting the deal, arguing that the county should only sell the facility and not the surrounding 200 acres of undeveloped land.