If a person wrote something on the palm of his hand before Shabbat, such as a phone number, and he did not wash the ink off his hands before Shabbat, may he perform Netilat Yadayim on Shabbat? Since the words or numbers will be erased from his hands as a result of the washing and drying, this would seemingly constitute a violation of the prohibition of Mochek, erasing on Shabbat. How, then, can an individual in such a case wash his hands for bread on Shabbat?
Chacham Ovadia Yosef addresses this question in his work Halichot Olam (vol. 4, p. 273), and he writes that a person in this situation should perform Netilat Yadayim by pouring at least a Revi'it – or 3.5 oz. – of water over each hand in a single flow. Halacha requires one to dry his hands after Netilat Yadayim only if he had not poured 3.5 oz. of water over each hand in a single flow. Therefore, a person with something written on his hands should pour a Revi'it on each hand in a single flow, so that he will not be required to dry them. Pouring water over one's hand without drying it will not necessarily result in the removal of the ink. It would thus fall under the category of "Davar She'eino Mitkavein," a case where an action is performed that could potentially result in an inadvertent Shabbat violation, but this result is not certain. A "Davar She'eino Mitkavein" is permissible on Shabbat where the result is not certain, and therefore a person in this situation should wash his hands in this manner without drying them, and he thereby fulfills Netilat Yadayim without transgressing the prohibition of Mochek.
Summary: If a person has something written on his hand on Shabbat, he should perform Netilat Yadayim by pouring on each hand 3.5 ounces of water in a single flow, and he should not dry them with a towel, but rather let them air dry.
