Ibn Abi Najih (may Allah have mercy on him) narrates from Ata (may Allah have mercy on him), Tawus (may Allah have mercy on him) and Mujahid (may Allah have mercy on him) that they said about the slanderer: If he repents, his testimony will be accepted.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20551