Tosefta do Kidduszin 1:13

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.

One can exchange land for land, moveables for moveables, land for moveables, moveables for land—when this one takes ownership, the other one becomes liable for its exchange (see Mishnah Kiddushin 1:6).