Do you sin by questioning the merchant in front of his customers, thus causing him to lose his livelihood, and did you wrong him by returning the device?

If the reason for the altercation was the shop owner's refusal to return a device for which he claimed a warranty existed, but it turned out not to, then there is no blame upon you for what you did. This is because if he was unaware, he is obligated to return it due to the absence of the agreed-upon condition. If he was aware, then it is deception. In both cases, it is permissible to expose him and make it known to people, based on the Prophet's (peace be upon him) saying: "The delay in payment by a wealthy person is injustice," and his saying: "The procrastination of one who is able makes his honor and punishment permissible." Therefore, it is permissible to warn against the danger of an oppressor who infringes upon people's rights and to expose him if he does not desist from his action, based on Allah's (the Most High) saying: {Allah does not like the public mention of evil except by one who has been wronged}. There is no sin upon you if customers turn away from him, because he is the cause, and clarifying the situation of one who insists on deception and trickery is permissible.