Tosefta על בכורות 61:10

Originally they (i.e., the Sages) said that [as to] a chaver that becomes a tax collector, we force him out of the chaverut [presumably, permanently], and then they reconsidered to say, that as long as he is a tax collector, he is not [even considered] trustworthy, [but once he] separates from [his office of] tax collector, behold, he is [considered] trustworthy. (Cf. Bekh. 31a:4.)