Probably not, assuming that all the other specs are in fact the same. mA is a measure of current, and the synth will pull what it needs. If the supply is unregulated, it might give a bit more voltage than it's rated for, but the synth is probably regulated internally, and hopefully built with a bit of tolerance for overvoltage.
Do double check that both are either AC or DC of the same polarity. DC (of either polarity) usually wont hurt a device that wants AC, though it might sound broken. AC, or DC of the wrong polarity can kill a DC device in seconds.