There is no backing out penalty of $1B, as there is no option of backing out. That penalty is only for extreme cases that cause the deal to fall through, such as a discovery of massive fraud or the financing falling through entirely (say, Tesla going bankrupt tomorrow). I those cases, Mush would stop owing $44B and onyl owe $1B instead to Twitter's shareholders.

Unless and until such a thing happens, he owes them ~$44B.