Old Biff meets young Biff in town and then drives to young Biff's house, where they leave the car in the garage. How does old Biff get back to the DeLorean?
Maybe by taxi. He used a taxi to get to the Delorean in the first place, then disintegrated as the fabric of time was woven. (This is in the deleted scenes section of the DVD.) Since he'd already used a taxi and would be investing spare change in getting millions of dollars, he wouldn't oppose taking the taxi again later.
The main flaw in this logic: the dates on the currency, as evidenced by Doc's briefcase of "all monetary possibilities". Using money dated 2015 wouldn't go over well in 1955. He may have written a check.
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