The reaction $R$ provided by the thief on the box of mass $m$ is, $R=mg-ma$. When the body is falling downwards freely, $g=a$. So, $R=mg-mg=0$. Hence, the thief will not feel any weight of the bag.
Home Gravitation Reasonings A thief weighing 60 kg jumps from the window of a house with a box of 40 kg in his hand. How much weight of the box will he experience?