Who was the Companion whom one of the polytheists slandered in the presence of the Prophet (PBUH) and Abu Bakr (may Allah be pleased with him), and who was the slanderer, and what was the name of the غزوة (battle/campaign) in which this incident occurred, and when?

What the questioner mentioned is true; the people of falsehood use various means to distance people from the truth, to discourage them, to belittle them, and to pressure them psychologically. An example of this is what Urwah ibn Mas'ud al-Thaqafi did with his nephew al-Mughirah ibn Shu'bah on the day of the Treaty of Hudaybiyah, when al-Mughirah was guarding the Prophet (peace be upon him). Urwah extended his hand towards the Prophet's beard, and al-Mughirah struck his hand and threatened to cut it off if he did not restrain it. Urwah was astonished and asked the Prophet about al-Mughirah, and he informed him that he was his nephew. At that, Urwah said to al-Mughirah: "O traitor! Was it not only yesterday that you washed off your evil deed?" referring to matters al-Mughirah had committed before his conversion to Islam, which Urwah had rectified.