The money will pay for 30 permanent teachers to get primary 1 classes below 30 this session.
Dr Mason, whose move was not seconded, launched an outspoken attack on "this stupid commitment" on class sizes which he regretted had been in the Liberal Democrat manifesto as well as Labour's.
"This represents a sizeable investment in relatively advantaged schools, where the problems of the largest classes tend to be. If we were to take this money and halve - not reduce - numbers in primary 1 classes in disadvantaged areas, I might be more convinced," he said.
"There are times when you should look gift-horses very firmly in the mouth and move them firmly on to the knacker's."
Malcolm Green, the city's education convener, betrayed some sympathy with Dr Mason but said the council should take the money and use it flexibly.
The most important element in raising standards was the quality of teaching. "A first-class teacher would do better with a class of 45 than someone who was second-rate who had half that number," Dr Green said.