The equation of motion tells you that a=0 when . This is the terminal velocity , the speed at which gravitational pull is exactly balanced by air resistance. If then the acceleration will be positive, and the velocity will increase towards . If then a<0 and the velocity will decrease towards .
In this problem, the initial value of v is 0, so v can never exceed . That is why you have to take to be negative. If the problem told you that the sandbag had been hurled downwards from the balloon with an initial speed greater than , then your solution would have needed to take to be positive.