The Chinese owners of paid around £5,000 a week for six months to lease a car linked to the wife of disgraced owner Carson Yeung.
A new report about the club's parent company Birmingham International Holdings (BIHL) reveals it forked out around HK$1.3 million - circa £106,000 - from July to December 31, 2014, for the vehicle.
The payment is revealed in the company's annual report.
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Football finance expert Peter Knowles said the arrangement, struck with a company linked to Yeung's wife Wang Man Li, formerly Wang Li Fei, suggested "the sort of agreement written on a fag packet".
The BIHL report states: "Wang Li Fei, having beneficial interest in Life Profit Asia Ltd, is a domestic partner of Mr Yeung, a substantial shareholder of the company.
"A motor vehicle with cross-border licence plates held by Life Profit has been leased to the company during the period.
"The group recorded rental expenses of approximately HK$1.3 million to Life Profit during the six months ended December 31, 2014, and the leasing service was terminated on January 10, 2015."
Mr Knowles said: "It is not clear what has gone on here. Why are they spending £5,000 a week to lease a vehicle?
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"It must have been a Rolls Royce or something very special for that sort of money.
"I wonder if they were paying out the money to allow her to visit Carson Yeung in prison? Who knows?"
Yeung, the club's former chairman, was jailed for six years in March 2014 after being convicted on five counts of money laundering involving a total of around £55 million.
Only last month, he lost his appeal against the verdict but still has a 27.7 per cent stake in BIHL.
The BIHL report also reveals that Birmingham City FC plc agreed with Peter Pannu's Asia Rays consultancy company a monthly rent of £5,000 for three years from March 1, 2011, to February 28, 2014, for an office in Hong Kong.
The report reads: "Asia Rays is wholly owned and controlled by Peter Pannu. The group recorded rental expenses of approximately HK$450,000 to Asia Rays during the period ended December 31, 2014."