Deceiving a Worker Suspected of Theft

May I leave money out to determine whether my cleaning help is stealing?

If one suspects their cleaning help of stealing they may be tempted to leave money in an obvious place to confirm their suspicion, and to catch the would-be thief red-handed.

The Ben Ish Chai writes, and other poskim concur, that the above ploy is only permitted if the owner declares the money hefker (ownerless). However, in a teshuvah (responsum) in his work, Torah Lishmah, the Ben Ish Chai ruled that doing so is absolutely forbidden—even if the money was made hefker

This is because the prohibition of lifnei iveir (placing a stumbling block before the unsuspecting) includes also causing someone to intend to sin and applies to unsuspecting non-Jews as well. There are other ways to find out whether someone is stealing, and to better protect one’s valuables; setting a trap is not permitted by halachah. 

https://halacha2go.com?number=549

Practical Halacha: One minute a day. By Horav Yosef Yeshaya Braun, shlita, Mara D'asra and member of the Badatz of Crown Heights.