For some were supposing, because Judas had the money box, that Jesus was saying to him, "Buy the things we have need of for the feast"; or else, that he should give something to the poor. [NASB]
They assumed Jesus was sending Judas to buy things they needed or to give something to the poor.
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The idea that Judas was the traitor must have been difficult for his disciples to believe. Judas was favored. He kept the money box. He sat in the place of honor. How could he be the traitor?