The news: A legal dispute between WPP and former executive Richard Foster has made public more than $9 billion in client ad spending data, including detailed breakdowns for Google, Coca-Cola, Unilever, and Ford.

The 35-page filing, submitted in a US court, reveals platform-level spending, revenue figures, and profit-and-loss information that agencies typically guard as confidential, per Ad Age.

The disclosure stems from Foster's $100 million whistleblower lawsuit, filed in November 2025 after WPP fired him after 17 years at the company. Foster alleges that GroupM (now WPP Media) retained media rebates that should have been returned to clients, reclassified that inventory as proprietary media, and resold it at a markup.

Why it matters: The WPP filing offers a rare look at how a major media holding company structures trading income, and it should prompt immediate contract reviews.

The court documents quantify those allegations. WPP Media generated roughly $1 billion annually in non-product-related income from rebate-linked arrangements, with internal growth targets of 15% YoY, per Digiday. Among WPP's top 30 US billing clients, representing $13.5 billion in total billings, 5% of eligible spend was routed through proprietary deals. Globally, Google accounted for $9.4 billion in spending, Meta $3.7 billion, and TikTok, Amazon, and The Trade Desk each accounted for $1.1 billion.

The case arrives as principal media buying accelerates across the industry. Some 81% of marketers say their firms plan to increase principal media buying in 2026, per MediaPost. Yet only 48% say they are very familiar with how principal media works, and 18% do not know whether it has been part of their media activity in the past year, per the ANA's principal media report. The ANA has urged marketers to require agencies to provide a clear business case when recommending principal media.

Implications for marketers: Brands that lack explicit language addressing principal media in their agency agreements risk limited visibility into how trading income is allocated. The gap between rising adoption (81% plan to increase principal buying) and low understanding (48% say they are very familiar) could create meaningful financial exposure.

The timing compounds the pressure. WPP's 2025 revenues excluding pass-through costs declined 10.4% as the company unveiled its Elevate28 turnaround plan, targeting £500 million ($670.25 million) in annual cost savings by 2028.

Media investment has continued to outpace agency revenue growth: Global ad spending rose 7.3% in 2023, 11.3% in 2024, and 8.6% in 2025, while average holding company revenue growth slowed from 5.2% to 3.0% and declined 1.2% in 2025. The widening gap has intensified pressure on holding companies to pursue incremental income streams, including principal media and rebate-linked arrangements.

As holding companies pursue new revenue streams to offset fee compression, principal media arrangements may become more prevalent, and marketers should treat transparency provisions as non-negotiable.

Brands should audit existing agency contracts for principal media clauses, demand full disclosure of rebate arrangements, and benchmark their spend against publicly disclosed platform-level data to assess where pricing assumptions are reasonable.