Found on Wikipedia, no. 3
Funny anecdote about what happened following the near-disastrous Apollo 13 mission:
Grumman Aerospace Corporation, the builder of the Lunar Module, issued an invoice for $312,421.24 to North American Rockwell, the builder of the Command Module, for “towing” the crippled ship most of the way to the Moon and back. The invoice was drawn up as a gag following Apollo 13’s successful splashdown by one of the pilots for Grumman, Sam Greenberg. He had earlier helped with the strategy for rerouting power from the LM to the crippled CM. The invoice included a 20% commercial discount, as well as a further 2% discount if North American paid in cash. North American politely declined payment, citing that they had ferried Grumman LM’s to the Moon on two previous occasions with no such reciprocal charges.
