in the case of the taxes imposed by sections 3101(a) and 3111(a) that part of the remuneration which, after remuneration (other than remuneration referred to in the succeeding paragraphs of this subsection) equal to the contribution and benefit base (as determined under section 230 of the Social Security Act) with respect to employment has been paid to an individual by an employer during the calendar year with respect to which such contribution and benefit base is effective, is paid to such individual by such employer during such calendar year. If an employer (hereinafter referred to as successor employer) during any calendar year acquires substantially all the property used in a trade or business of another employer (hereinafter referred to as a predecessor), or used in a separate unit of a trade or business of a predecessor, and immediately after the acquisition employs in his trade or business an individual who immediately prior to the acquisition was employed in the trade or business of such predecessor, then, for the purpose of determining whether the successor employer has paid remuneration (other than remuneration referred to in the succeeding paragraphs of this subsection) with respect to employment equal to the contribution and benefit base (as determined under section 230 of the Social Security Act) to such individual during such calendar year, any remuneration (other than remuneration referred to in the succeeding paragraphs of this subsection) with respect to employment paid (or considered under this paragraph as having been paid) to such individual by such predecessor during such calendar year and prior to such acquisition shall be considered as having been paid by such successor employer;