The deal the government carved out with Labour backbenchers to get the foundation trust legislation through Parliament has precipitated a battle between the trusts and Unison over the limits of their freedoms (for more background, click here).
Tony Blair appeased opposition to these landmark reforms by imposing a statutory cap on the trusts' income from private patients. This limited private income to the same proportion of their total income as it was five years ago.
At the time it no doubt seemed like a price worth paying, but the ramifications are immense.
Several foundation trusts - true to their entrepreneurial aspirations - exploited a loophole in the rules to establish separate operations or joint ventures providing private services, then channelled the money back to the trusts' coffers.
Unison has now taken exception to these legal gymnastics, and has threatened regulator Monitor with a judicial review if it does not make the cap stick.
Former health secretary Frank Dobson, always a private sector sceptic, advances the compelling argument that a regulator does not have the right to undermine the democratic decision of Parliament.
And however laudable the motive - generating income to fund services - the ruse is against the spirit of financial transparency that should underpin public sector transactions.
But ministers need to openly discuss a solution. Locking income to arbitrary figures plucked from five year old accounts makes no sense. While there is something appropriately symbolic in the opponents of foundation trusts pinning their hopes on a bygone age, it is not a sensible way to run a health service.
If Monitor is pushed into imposing more puritan interpretations, several trusts could lose income of millions, and many more will be deterred from developing these services. Great Ormond Street Children's Hospital, for example, says it will lose£8m a year once it secures foundation status if the rules are tightened.
The Foundation Trust Network has rather meekly said that primary legislation to remove the cap is unrealistic.
The best hope for a short-term palliative appears to lie with ministers 'redesignating' what comes under the cap.
But this smacks of simply finding another loophole to replace the one currently being kiboshed by Unison.
Sooner or later Parliament will have to revisit the issue.