Musannaf Ibn Abi Shayba 13:3242

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, from ‘Amr, from ‘Abd al-Rahman ibn Farrukh: That Nafi‘ ibn al-Harith bought a house for a prison from Safwan ibn Umayyah for four thousand dirhams. If ‘Umar is pleased, then the sale is for him, and if ‘Umar is not pleased, then four hundred [dirhams] are for Safwan.