Irwin Gotlieb, the global chairman of GroupM, the world's largest media buyer, told a conference this week that agency trading desks — the digital platforms that buy online ads — need not be completely transparent with their clients.

The issue sounds dull but, financially, it isn't. GroupM (a unit of WPP Group) handles $73 billion in media billings annually, of which $6 billion is digital spending.

The words "agency trading desk" and "transparency" are hot-button issues in the ad world right now: Some clients — and many ad-buying competitors — insist that agency trading desks are run in such a complicated, conflicted, non-transparent way that clients do not know exactly how their money is being spent, and may therefore get ripped off. One accusation is that when a media buying agency allocates part of a client's budget to an agency trading desk owned by the same company, both the buying agency and its sister trading desk take a fee for doing so — which looks like double-dipping.

For their part, agencies say that sophisticated clients don't tolerate such maneuvering and that fee schedules are contractually disclosed.

So it's interesting to hear Gotlieb go anywhere near the issue. Here's what he said, according to AdExchanger. He was asked how trading desks can be fair to clients:

"Transparency is of the essence. Having said that, it doesn’t say in Genesis that everything we do has to be on a fully disclosed basis to clients. We compete with parties that are equity funded where profitability isn’t an issue. We must develop technology, create structures to exploit it for the benefit of our clients."

Gotlieb has caused controversy on the "fully disclosed" issue before. In 2008 he gave an interview to BusinessWeek in which he appeared to confirm that he favored taking volume discounts from media suppliers in return for ad buying contracts. Such discounts are controversial because they can be lucrative — 10 percent of a client's budget is a common rate — and clients sometimes don't know they exist, even though it's their money that earns them. If agencies keep such discounts without telling clients, it could create legal liability.

To be fair, he did NOT say GroupM was actually taking such discounts, and he disputed the accuracy of BusinessWeek's reporting at the time.

The volume discount issue came up again recently when it emerged they played a role prior to the collapse of Leo Burnett in Greece. Infamously, Interpublic Group in 2008 settled accusations from the SEC that it booked $250 million in clients' volume discounts as its own revenue.