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A =  d v / dt  = (v2 - v1 ) / t

if v2 < v1,  then  a is negative.  always....

Only possibility that    a  can be positive   is that you choose a sign convention in such a way that  quantities in the direction of movement  are negative.  and the quantities in the opposite direction are positive.

Choose  the downward direction (vertically) as the positive  y axis.   If a stone is throne upwards with an initial velocity  u.  Then the displacement s = 0 at t = 0 and then it becomes more and more negative.  Hence,  velocity will be negative.    As the speed reduces,    change in velocity is  positive  ie., acceleration is positive.

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