each of us deserves to be not only treated with respect but also valued as a part of humanity whatever our origin or our place in society
That may be true, for at least three reasons:
It's not "a person's place in society" that makes a person valuable: for example verse 396 in the Dhammapada (there are other examples in the suttas) say that it's not a person's caste (place in society) that would make them holy.
Buddhism teaches that being born human is of itself valuable (because humans are able to work towards enlightenment, whereas being born an animal makes that more difficult or impossible).
Compassion and loving-kindness is arguably a (if not the) prime motive of Buddhism.
Also, and I'm not sure whether this counts, at least in its original form Buddhists were encouraged to renounce the ties of having close family and the responsibilities of having a household to support -- in exchange, I think that monks and nuns acquire broader or more universal social relationships, i.e. to the whole Sangha and to (all) members of lay society.
To be fair I can also think of some Buddhist beliefs (not that all Buddhist have these beliefs) which might be counter-examples to your "(valued as a) part of humanity whatever our origin" definition:
- It's better to be born in a country/society where the Buddha-Dharma exists
- It's better to give Dana to the Sangha than to other people
- One should avoid being too anthropocentric (seeing people as special just because they're human), and one should consider animals and so on too under the category of "sentient beings"
- The Sigalovada Sutta (which describes "the six directions") says to behave differently towards different members of your society depending on their 'place' (though maybe each with dignity in their own way)
- Buddhist doctrine might identify (shine a light on) human foibles (a.k.a. "defilements"): greed, anger, laziness, ignorance, and so on. To that extent maybe it doesn't assume that humans are all always especially dignified all the time.
These 'counter-examples' don't necessarily mean that Buddhism has no concept of human dignity; but if these contradict your definition of human dignity, then maybe it wasn't a fool-proof definition.
I read that each of us has the nature of Buddha. Could this be considered as the foundation of Human Dignity or a similar notion in Buddhism?
Yes, I'm pretty sure it could.
The Buddha deserves respect so by recollecting the Buddha one feels respect (IMO).
The Buddha taught (or Buddhism teaches) people to behave themselves -- minimally the five precepts e.g. don't kill people, don't lie to them or steal from them -- or in more detail, much advice on how to talk respectfully.
The "nature of Buddha" is (it's written) difficult to define. Part of its meaning is, apparently, the "seed, embryo, or womb" of Buddha, i.e. the potential for being Buddha.
Speaking of potential etc., my opinion (and this isn't necessarily Buddhism) is that something like this is an argument for, for example, treating children with dignity. You may prefer relationships with people that are not stained by anger, greed, coercion, nor indifference.
Being a little bit disrespectful of people's vanities might be enlightening sometimes (see e.g., I don't know, the Calling Card or the Nothing Exists stories for examples of that), but doing that kind of thing deliberately is maybe something that only a teacher should do.