Tosefta על קידושין 1:2

Which is "deduction of price" [through which a Hebrew slave can go free according to Mishnah Kiddushin 1:2]? He wanted to redeem himself within those [6] years [that he is a slave], he calculated the coins and the years [remaining] and payed them to his master, and the slave has the upper hand [he can calculate based on how much his price was worth when he sold himself originally or how much that would be worth now, whichever is better for him]. Which is presumption [of ownership] of land? A lock, fence or opening of any size [that belongs to him]—this is a presumption [of ownership]. Which is presumption [of ownership] of [Canaanite] slaves? If [the slave] ties [the owner's] shoe, undoes his shoe, brings vessels after him to the bathhouse—this is a presumption. If he lifted him up [e.g. to help him get on a horse]—Rabbi Shimon said: There is no greater presumption than this.