Tosefta zu Schemot 21:26

[The Hebrew slave] acquires himself with [loss of] major limbs (see Shemot 21:26). He acquires himself with money via others and a contract via himself, [and he can't acquire with his own money] because he is like one who gives from the left hand to his right [i.e. whatever is his is his master's, so the money needs to come from elsewhere]—words of Rabbi Meir. But Hakhamim say: [Even] money via himself or a contract via others, as long as the money belongs to others and he says to him [when he gives him the money], "[This is] with the understanding that you can only redeem [yourself] with this money". Rabbi Shimon ben Elazar said in the name of Rabbi Meir: Even a contract via others, but not one via himself.