וְכֶ֥סֶף מִשְׁנֶ֖ה קְח֣וּ בְיֶדְכֶ֑ם וְאֶת־הַכֶּ֜סֶף הַמּוּשָׁ֨ב בְּפִ֤י אַמְתְּחֹֽתֵיכֶם֙ תָּשִׁ֣יבוּ בְיֶדְכֶ֔ם אוּלַ֥י מִשְׁגֶּ֖ה הֽוּא׃
and take double money in your hand; and the money that was returned in the mouth of your sacks carry back in your hand; peradventure it was an oversight;
Radak on Genesis
וכסף משנה, and a second amount of money. The word משנה occurs in the same sense as here in Jeremiah 17,18 ומשנה שברון, “and shatter them with double destruction.” The vowel segol instead of tzeyreh in שני, indicates that the word is not in a construct mode.