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Chosen answer: It only took me a couple of seconds to find a fan site with a credible answer. Apparently, in an early draft, Marty mentions in passing that Doc hired him to clean out his garage for fifty bucks and total access to his record collection. This scene was presumably never shot, thus the backstory was lost to time. See www.kristensheley.com/bttf/bttfuniverse.html for more extensive info.
Macalou