Assalaamu alaykum. Is it permissible to eat at a place that makes sandwiches where the workers are supposed to use gloves and change them for each customer, even if a worker may have not done so and there may have been contamination of a non-halal ingredient on a halal ingredient? Is one sinful if they eat in such as place?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

The fact that the workers do not change the gloves does not make the food that they make unlawful. Mere doubts that the ingredients of the food may be mixed with an unlawful substance does not forbid eating it. However, if this restaurant mostly uses some unlawful ingredients in their food, then one should ask about the ingredients of the food and make sure that it is not mixed with any unlawful ingredient.

Shayk Ibn ʻUthaymeen said, "So long as we know that most of the food that is fried with this oil is impure, such as dead animals or pork, then we have to ask. However, if we do not know whether most of the food fried with this oil is impure or other than that, then it is not obligatory to ask."

Allah knows best.