You shall not return a slave to his master, if he flees to you from his master. He shall dwell with and among you, in the place which he chooses, in one of your cities, as is good for him; do not wrong him. (23:16-17)
"The place which he chooses" is a familiar phrase to anyone who has been reading Sefer Devarim the past few weeks. It is used 10 times in the book to refer to Jerusalem (or Shiloh) - the location which God would eventually indicate for the Temple, but which was not yet known in Moshe's time. Here the same phrase appears, referring not to God's choice of Jerusalem, but of the slave's choice of a place to live.
What's the reason for the use of identical (and distinctive) language in this totally different context? I don't really know, but perhaps this teaches us to welcome the runaway slave as enthusiastically as we would welcome the Temple (before we knew where it would end up being).
Hashem has called you an abandoned and distressed wife. "Can a wife of one's youth be rejected?" says your God. "For a small moment I abandoned you, but with great compassion I will gather you [to me]." (Yeshayahu 54:6-7, from the haftarah)
The haftarot between Tisha Beav and Rosh Hashanah are all from Yeshayahu, and all of them seem to be chosen based on their relation to the time of year, not their relation to the parsha.
But this week, it is clear that there is a relation to the parsha as well. The haftarah is centered around the extended metaphor of Israel as God's abandoned wife. And the parsha is packed with laws about women and marriage (1). Thus, out of all the possible haftarot which fit the theme of the time of year (anything in the 2nd half of Yeshayahu, I'd assume, plus some other possibilities), the one which fit our parsha best seems to have been chosen.
1. For example: the war-captive wife, the loved and unloved wives, the various laws of adultery and virginity, incest, marriage with genitally damaged men, marriage with converts from various nations, prostitution, divorce and remarriage, that newly married men don't fight wars but rather gladden their wives, and yevamot.
UPDATE: In the "Daf Kesher" newsletter for Parshat Ki Tavo 5768, R' Amnon Bazak notes the same thing about the runaway slave passage that I did, and comes up with a different, perhaps better, reason for why it's phrased that way.