What was said by our Master and Rabbi Samuel de Medina, that “This custom is learned from foreigners…and we are to follow their custom” clarifies that customs of merchants may annul Halakha. There is always something to learn, even from the widespread customs of non-Jews. See in HaKnesset HaGedola, read Mateh Shimon and read Mekor Baruch, where it is written that in all that concerns laws governing money, custom is foremost and even the customs of non-Jews are learned by Jews.