German preacher: «A Muslim cannot work in a supermarket.»

No. A person should not work in a supermarket where there are mainly two jobs. These jobs are almost always stocking shelves or working on the tills. There is one exception, which I will explain later.
When someone restocks the shelves… What do they put on them? Only halal items? No. They also put on forbidden products, such as alcohol, meat that isn't permitted, gambling (for example, lotteries you can buy at the till), cigarettes and so on. All of this is forbidden.
And it is not enough to say, “I don't drink,” “I don't smoke,” “I don't eat pork.” No. The Prophet Muhammad —peace be upon him— said: when Allah forbids something, He also forbids its price.
This means you cannot buy it, sell it or store it. And even less can you make it easier for someone else to buy it through you. Because if you do, you will share in the sin.
That is why, brothers, it is forbidden to work in a supermarket restocking shelves or working at the till. What does the cashier do? He charges for these products, and thus the person can take them away. This means that here too he has actively helped them to commit the sin.
The only exception is if, in a large supermarket, one person is solely responsible for the fruit and vegetable section and has nothing to do with the other sections. In that case it would be allowed. But this is very rare..