Environment Canada records show that there have been just three 20+ mm rainfalls on July 24 in Halifax over the last 50 years---in 2006 (28.7 mm), 1988 (36.5) and 1965 (35.8mm). But those were all-day totals; Environment Canada doesn't give hourly totals for historic rainfalls, but it seems unlikely any of those three rains would meet Power Events' payback requirements. It's unlikely that even Hurricane Juan dropped 20 mm in three hours---Sept. 28 and 29, 2003 all-day rain amounts were 38 mm and 30.8 mm, respectively.
One historic 20 mm+ three-hour rain was Hurricane Beth, a storm of biblical proportions---the elevator shaft at Fenwick Tower was filled with four metres of water, Dartmouth's Lake Banook overflowed into the Duck Pond and then down Portland Street---which on August 15, 1971 dumped a whopping 185.2 mm on Halifax, the biggest one-day rainfall ever locally. On average, that's 23.2 mm over each three-hour period, which *barely* meets Power Promotions' guarantee.
But don't get your hopes up, concert-goers: as of this writing, there are no hurricanes on the map and Environment Canada says Saturday will be a pleasant 21 degrees, with no rain.