I once was told that the Haida practiced a form of slavery in which men captured in battle would be enslaved for a period of 7-10 years, at the end of which, they would be set free with a celebration. However, this sounds suspiciously close to the practice of the year of Jubilee in the Old Testament, so I was wondering where the truth lay.
The seminal document on Northwest Coast cultural practices with regard to slavery is Leland Donald's Aboriginal Slavery on the Northwest Coast of North America. I also like to use early ethnographic accounts such as John Swanton's 1905 Ethnology of the Haida. I looked at these and other resources and could find nothing resembling the practice you describe.
Indigenous slavery was a relatively common practice from Northern California to the Arctic. Most of the people that practiced slavery treated slaves as a commodity. They were traded extensively and often used to pay debts or purchase valuable items including wives. Slaves were acquired by raids, trades, or heritage (children of slaves were slaves). As noted above, the Haida were notorious as slavers. One estimate had it that over 25% of Haida were slaves (Donald pg. 186).
I have seen references of slaves being able to buy their way out of slavery, although it was rare and in the southern NWC groups. I can find nothing about slave owners freeing slaves after a set period of time.
Edit: fixed a reference and grammar
I haven't come across that specific timeline in my admittedly limited reading about slavery in the Pacific Northwest. If you have access to JSTOR, there is an article titled Predatory Warfare, Social Status, and the North Pacific Slave Trade that is pretty good and briefly mentions the Haida in particular. I also have a print copy of Peoples of the Northwest Coast: Their Archaeology and Prehistory by Ames and Maschner.
My understanding is that the Haida were particularly well known as slavers in the region (some scholars in the field liken them to the Vikings of the Pacific Northwest). Also, high status individuals such as chiefs had the power of life or death over slaves, and slavery was hereditary in many groups. The value of slaves was such that they were seen as one of the most ideal potlatch gifts. Essentially, an individual planning a potlatch down the road could trade for some slaves ahead of time. These slaves would, of course, produce more goods for the owner to then give at the potlatch, so the slaves were an excellent investment in these regards. For these reasons, I find it likely that a strict 7-10 year limit on being a slave either did not exist or was not widely practiced. I find it even more unlikely that a celebration was held after freeing a slave given the stark contrast between slaves and freemen in these societies as well as the value of a slave.