It sounds as though the spiteful merciless manager may have been out to get this poster. That's not illegal. Our employers and supervisors are not there to help us fulfill our dreams of home ownership or anything else.
The OP handed said manager a freebie, a "Please, take this perfectly legal and wonderful reason to fire me for a dramatic violation of company policy, since you want to anyhow." (And probably not have me qualify for unemployment benefits, either).
For minor violations of "the rules" as they apply at your particular place of business, it is usual to give warnings and write ups before termination. However, there is no labor law that requires that a company do so. You live in an "at will" state, and you can be fired at will by your employer. Your only recourse at this point and in this situation is to file for unemployment insurance benefits. They will look at the reason for your termination and make a determination if the employer had a valid work related misconduct reason to terminate you. Then either party can appeal this determination.
If the violation was egregious enough in the eyes of the unemployment system, it is classified as "gross misconduct" which means something so wrong that the employee did not need to be warned, even once, that they were NOT to ever do this, that a reasonable person would have known that doing this, even once without being warned would likely result in their termination. Classic examples of this are stealing money from the employer, punching out a co-worker, setting the workplace on fire, etc. No unemployment is forthcoming in that type of situation.
This one is sort of on the fence, as the employee does a brilliant job of poor me-ing and playing completely innocent of any expectation that this could be a problem or that it was not acceptable to do this because both parties were known to everyone and he was making no secret of what he was doing. He did, after all, give the deal to the manager for approval without any effort to conceal his actions or seeming expectation that it wouldn't be appropriate. The unemployment adjudicator would probably probe in the direction of what the company's policies were, what others had done in the past (had the OP ever seen anyone else do a transaction of this type for a family member or friend?) and what was said during the termination.
But I suspect that the OP knew totally and fully, exactly what the rules were, what the deal with doing this was, and he wrongly assumed it would be all right for him to do it anyway, since, after all, it was his fiancé and everyone who worked there KNEW it was, and knew (supposedly) they're on good terms and she had full knowledge of what was going on. That was dumb on his part, and he says he did it simply because it would have been "inconvenient" for his fiancé to come and get the check herself. Where I come from teachers sometimes are able to get off work sooner than banks close.
The OP works totally from a point of view that the rest of the world should be concerned about his financial situation, his emotional situation, his needs and wants and his and his fiancé's convenience and should be willing to make an exception to very strict bank policy based on "Aw, come on guys! This is me!" He's not so much a criminal, there was probably no criminal intent here, as he's sort of selfish and oblivious to any rules except those that benefit him.
It probably will be possible for them to get out of the house buying deal at present, but they'll certainly lose money. This is unfortunate for them. But this is not the fault of the bank manager. I doubt if the business will be devastated by his fiancé's closing out her account, either. These were the rules. You broke them. Someone who perhaps did not like you anyway enforced the rules as they were supposed to anyway. They did not make a special exception for you.
Unemployment insurance is a maybe. Moving forward a little sadder and wiser is inevitable.
