NHS Tayside has been warned that using charity funds to buy basic equipment should only be a "one off".
The charity watchdog found rules had not been broken
The caution came from the Office of the Scottish Charity Regulator (OSCR) after up to £400,000 of donations was used to buy nurses' uniforms in 2004.
However, OSCR ruled that the purchase of the garments was permitted because they improved staff working conditions.
The organisation also said that it was not in the public interest for its inquiry to continue any longer.
The findings were contained in a letter from Tom Thorburn, a senior investigations officer at OSCR, to a former member of Tayside Health Board who had raised concerns about the use of charity funds to buy uniforms.
The complainant, Bill Stewart, felt that the money should have come from the mainstream NHS budget.
Mr Thorburn's letter states: "I have no doubt that the purchase of these uniforms was permitted by the objects of the charity which inter-alia include 'Motivation of staff, by improving staff facilities and by providing services that improve staff well-being.'
"The purchase of the uniforms, which were an entirely different style to those they replaced and almost certainly did improve the nurses' working conditions can be seen to fit into the objective I quote above."
Later in the letter Mr Thorburn added: "While it was OSCR's opinion that the purchase was intra-vires we needed some form of assurance that the trustees were aware that a purchase of this nature could only really be seen as a one-off event and that basic equipment should be provided by the health service and not by benevolent donors."
He also raised questions about the fact that only two trustees attended the meeting at which the decision to buy the uniforms was made.
Mr Thorburn said that although that was within the rules, decisions involving major expenditure of charity funds should normally be made after consideration by a substantial number of trustees.
Professor Tony Wells, chief executive of NHS Tayside, said: "As we have previously stated, OSCR wrote to us in December last year when they had completed their review and advised us that they were satisfied with our decision and they would be taking no further action.
"Therefore, I am surprised and concerned with the content of the letter Mr Stewart received and, as such, I will be seeking urgent clarification from OSCR."