The start of Mishpatim (Exodus 21:2-6) discusses the Hebrew slave. It starts with "Ki sikneh eved ivri" - "When you will buy a Hebrew slave", referring to the master in the second person. The remainder of passage uses the the third person for the master, as in "Im adonav yiten lo isha" - "If his master will give him a wife". Why the change?
Sifrei says in Deuteronomy Re'eh:
Ohr Hachaim explains very similarly in Exodus on our verse (The Birchas Shimon, who comments on the Ohr Hachaim, points this out, citing the Sifrei):
It seems like the direct language is being expounded upon, as opposed to a more incidental 'If a man shall buy a Hebrew slave...'
This interpretation is further reinforced, in my opinion by the cantillation marks. Over the word, תקנה, you shall acquire, we find a Pashta, a distinctive mark of the second degree, meaning that it enforces a moderate pause in the verse, making it read like so (number of commas indicate strength of pause):
That pause could cause the verse to be read: