פירוש על קידושין 32:20

The Torah directly states that one who goes free at the end of six years receives the gift. Through the repetition of the verse, the baraita adds the three other circumstances that according to the first opinion entail a gift from the master—Jubilee, master’s death and “signs” for a female slave.
This baraita excludes the runaway and one who buys his freedom. They are not “sent away”—they leave on their own.

To R. Meir, a runaway is not sent away and therefore does not receive a gift. But one who buys his own freedom is sent away, even though he can force this to occur. Therefore, he too receives a gift.