At the time, Tesla's Model S was new and buggy. Tesla was losing sales, and it was looking like it might go bust.

Google was Musk's savior.

From an excerpt published on Bloomberg:

In the first week of March 2013, Musk reached out to [Google chief Larry Page], say the two people familiar with the talks.

By that point, so many customers were deferring orders that Musk had quietly shut down Tesla's factory.

Considering his straits, Musk drove a hard bargain.