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Selling Tzitzis to a Gentile: 
One may not sell a Tallis with Tzitzis to a gentile, even if the gentile is a merchant, as doing so may lead to murder. Likewise, one may not give a gentile a Tallis with Tzitzis as collateral, or as an item for him to guard, unless it is for such temporary basis that there is no worry that a Jew may escort the gentile on the road in the interim. [Some Poskim rule that in today’s times since the gentiles are no longer suspected to commit murder, this law no longer applies. Other Poskim however rule this law applies even in today’s times, as we have unfortunately witnessed horrific acts of terror that have occurred under disguise of religious men wearing Tzitzis. It is thus incumbent on Jews to not allow gentiles to have access to Tzitzis, and certainly they are not to be given or sold to them.]
It is forbidden to sell a pair of Tzitzis to a gentile, or give them to him as a present or collateral.
If one sees a gentile with Tzitzis is he to try to purchase it from him?
Yes. Some Poskim write it is an obligation to purchase it from him in order to prevent any possible danger from occurring. This certainly towards gentiles that are known to be hostile towards Jews.
If one sees a pair of Tzitzis in the open is he to take it in order to prevent a gentile from taking it?
 20/6; Menachos 43a; Michaber 20/2
 The reason: As perhaps the gentile will wear it and appear like a Jew, and a Jew who sees him will think he is Jewish being that he is wearing Tzitzis, and the Jew will thus join him on the road. This can lead to his murder by the gentile, as the gentiles are suspected for murder. [ibid]
 Chayeh Adam 11/42
 Kaf Hachaim 20/12
 Lev Chaim 3/1