There are many examples of Germans rescuing Jews during WWII. Are there similar examples of slave owners doing the same during the 18th and 19th centuries? (e.g. buying slaves in order to set them free)

I would begin by saying that the example you give isn't quite on par with the rescuing of Jews from the Nazi regime; the thing about buying slaves to free them is that, whilst a noble idea, it is ultimately an act that legitimises slavery. By purchasing a slave, an abolitionist would effectively be doing two things - first, they would be acquisicing that the institution had a legal basis with which they were prepared to accept and engage (i.e. that it was possible to own property) in the interim until abolition. Second, they were contributing to the financial system and rationale that helped to underpin and sustain slavery. In the case of buying slaves imported from Africa, this obviously meant providing support to the transatlantic slave trade, which could only exist by virtue by the fact people were willing to pay money for their Human cargo.

For owners though, selling a slave was not akin to selling, say, jewellery today. Indeed, it isn't really like selling anything today. In slave-holding societies, slaves become more than just labour; they became a form of uniquely self-replicating capital. Like any asset, slaves depreciate in monetary value over time (as older slaves inevitably become worn out physically and mentally by the relentless experience of slavery) - unlike other assets though, slaves are capable of creating new, equal or even more valuable assets (i.e., their children, who became the property of the slave owner). So slavery's economic appeal transcended the profit that came from having such an easily controllable, cheap labour force - it could also be a very profitable and secure way of investing money. Selling a slave represented the liquidation of a capital asset as much as anything else; thus generally speaking, slaves being sold on within the USA were surplus to requirement, their owner looking for a return on investment (which may then be used to purchase new slaves to quickly expand their workforce or spread the burden of their investment). Buying slaves from owners looking to sell also meant providing support for an important and equally insidious aspect of the slave institution. Though doing so would have benefited the individuals freed, it ultimately also meant providing sustenance to the system that kept slavery alive.

Having said that, it certainly is true that some early abolitionists experimented with doing this (and in a similar vein, many free blacks purchased their own freedom, or that of family members). No element of white society however was more weary of the idea of a free black population thriving in America than the plantocracy of the antebellum south; as you might expect, they took a particularly dim view of the prospects for free blacks to make themselves productive and worthwhile members of society, and had profound concerns that they might become a powerful force to agitate for insurrection or abolition. As such, throughout the antebellum period, legislation was constantly being constructed and refined to make emancipation through purchase extremely difficult or impossible. Indeed, by the late period, it was difficult even for legitimate slave owners to free their slaves. For example, from 1801 to 1815 there was no legal mechanism in Georgia by which slaves could be freed without the consent of the state legislature; from 1815 to 1859, manumission was only possible if the owner died and had bequeathed freedom to their slaves in a last will and testament. This law, however, also invalidated any last will and testament that dealt chiefly or exclusively with freeing slaves.

As such, this was an experiment that was short lived. Not only would many anti-slavery agitators have felt profoundly ill-at-ease that buying slaves meant providing support for the slave system, but the white establishment was very keen to restrict the capacity of even 'proper' slave owners to free their slaves. This was never a meaningful way to try and achieve abolition.