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:
מנין כשאתה קונה לא תהא קונה אלא עבד עברי שנאמר כי תקנה עבד עברי
From where [do we know] when you buy [a slave] you should only buy a Hebrew slave? As it says: 'When you shall buy a Hebrew slave.
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):
Perhaps that it means to say that if it will be before him to buy a Canaanite slave and a Hebrew slave, he should prioritize the Hebrew, and that is the measure of the verse, when you will want to buy a slave, prioritize buying a Hebrew, and don't say 'I will buy a slave that I will work with him forever, and I will not buy a slave that after six years I will send him off'...
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):
,,,,כי תקנה,, עבד עברי,,, שש שנים,,, יעבד,,,, ובשביעית,,, יצא לחפשי,,, חנם
When you shall acquire ,, a Hebrew servant ,,, six years ,,, he shall serve ,,,, and on the seventh ,,, he shall go free ,,, free of charge ,,,,
That pause could cause the verse to be read:
When you shall acquire [a slave, it should be] a Hebrew slave