I guess in hindsight you wish you had gone to your boss to denounce the customer's behaviour and put forward your poor handling of the situation for complete transparency prior to the complaint. Unfortunately, this starts you off on the wrong foot, but let's break it down a little...
The customer was clearly agitated before you dropped the f - bomb, so there is a very good chance his complaint has nothing to do with your closing remark or even if he heard it on your departure. On the other hand, he was annoyed at something you were (or weren't) doing and this is more likely to be the basis of his objection - ie, not receiving the customer service to which he felt entitled (rightly or wrongly).
'Worst case scenario' is my general approach too when I stumble - and it is always never that bad in reality. This just helps to mentally prepare (and emotionally torment us) for the lesser impact in comparison. Right now you are needlessly creating scenarios out of scraps of information - you will have your chance to present your side of events and defend your actions if you feel they were justified.
My best advice to quell worry is to recreate and write down every instance of the encounter (objectively please) to see what elevated tensions for both parties. You might see he was unreasonable or even that you made a bad decision - it is what it is. Once you have a clear picture, it must be left up to the team to weigh up your actions and past performance to ascertain validity or otherwise. How would they have handled this situation in your shoes? How will you respond respectfully and in support of the complainant to find understanding? It's best not to factor in the promotion as it could easily cloud your judgement and lead to distortion of fact.
Keep it real and prepare the background work, as how you conduct yourself now may be more pertinent than you think.
