Seeing this topic reminds me of a brain teaser I saw recently, which said that three sheep fell into three tree holes respectively. Why did the wolf choose to eat the middle? The answer is because wolves are willing. Sometimes we think the problem is too complicated, but it is not as simple and straightforward as children. Maybe there is no special reason why Yin let Lao Zi go through customs.
Hangu Pass is one of the important checkpoints in the Qin Dynasty. Its main purpose is to limit the flow of population in peacetime. Therefore, the general who guards the customs has great power, and all businessmen need to bribe him. And you must have a customs clearance certificate issued by the state to clear customs. In fact, there are many people who enter and leave the checkpoint privately, and there are many ways to avoid this situation. Usually, the inspection at the checkpoints is not strict, and it is not a serious violation for the supervisor to let one or two people settle the case.
If we have to analyze this matter, it may really have something to do with that legend. It is said that Yin likes reading and divination. One day, when he saw the purple gas coming from the east, he judged that there would be a saint going through the customs, so he stopped Lao Zi and asked him to write a moral classic, and let Lao Zi go smoothly.
In fact, Lao Tzu was also a court official in the Zhou Dynasty. Although his position is not high, he belongs to the noble kind, similar to the librarian of the National Library now, with no real power but great influence. Laozi didn't have the opportunity to directly participate in state affairs, and his comrades were not very keen on him, but at least he spent so many years in the central court, and his interpersonal relationship could not be ignored.
Yin is the CEO of Hanguguan. Although he is not a big official, his position is very important, and it is very likely that there are people on it. Moreover, he likes reading books himself, so he is likely to have an intersection with Lao Tzu, and he may have a good relationship before, so it is not impossible to let Lao Tzu go in order to be an official with the DPRK.
As for the so-called purple gas coming from the east and going out of Hangu Pass from the west, there is no way to verify it. No one knows whether this is true or not. Let him go in a fog. Anyway, it has become an allusion now. Let's take this legend as true. Yin really thought Lao Tzu was a saint, so he used the Tao Te Ching to get through and let Lao Tzu go west.
No one knows where Lao Tzu went after he went out of the Buddhist paradise. It is also said that he spread Buddhism in India and learned about Siddhartha Gautama. Who knows what real history is like? Consider it a joke. There is no need to study it too much.