Another possibility, of course, is that the bio-mimetic gel never left Starfleet's hands. We know Sisko agrees to the deal, but once Tolar dies, assuming the gel was ever actually delivered to him in the first place, it doesn't necessarily follow that it falls into Garak's possession...and if it did, I'd think Sisko would have another "conversation" with him, particularly if the gel could point the way to Federation involvement in Vreenak's demise.
I think there's also some question as to whether Sisko would ever let the gel enter Garak's possession in the first place. Yes, he was playing middleman, but considering how controversial the substance was, Sisko might have insisted on it being transferred directly to Tolar. (N.B. I don't remember the details of any pertinent dialogue.)