Calling up someone for Maftir if he does not know how to read the Haftorah:
The dispute in Poskim: Some Poskim rule that it is permitted to call up for Maftir an individual who does not know to read the Haftorah, and then have another individual read the Haftorah [with its blessings] in his stead. Other Poskim, however, rule that it is forbidden to do so, as the person who receives the Aliyah of Maftir is required to read the Haftorah.
The final ruling: Practically, it is initially forbidden to call up for Maftir an individual who does not know how to read the Haftorah himself. However, Bedieved if he was already called up, he is to say the blessings of the Aliyah and have another read the Haftorah [with its blessings], as rules the first opinion. [In such a case, another person may read the Haftorah with its blessings, even if he did not receive any Aliyah to the Torah that day. If the Maftir already said the before blessings of the Haftorah and it was then discovered that he cannot read the Haftorah, then another person is to take him over and the before blessing is not required to be repeated. However, initially the Olah for Maftir should not say the blessings of the Haftorah if he will not be reading the Haftorah. Alternatively, instead of having another person read the Haftorah instead of the Olah for Maftir, one is to appoint an expert individual to quietly read the Haftorah to the Maftir who does not know how to read, and have him say it out loud, repeating after the words of the expert silent reader. Some Poskim, however, rule that the above debate only applies in communities who do not read the Haftorah from a scroll. However, in communities which read the Haftorah from a scroll, it is even initially permitted for them to call up someone who does not know to read the Haftorah, have him say the blessings and have the Baal Korei read it from the scroll, just as is done by Kerias Hatorah. Furthermore, some Poskim rule that in communities in which it is accustomed for all the congregants to read the Haftorah along silently with the reader, then it is permitted even initially for the person who received Maftir to recite the blessings, and read the Haftorah quietly having the Baal Korei read it aloud. Practically, this is the custom in many communities, and they are not particular to call up only someone who knows how to read the Haftorah.]
Initially, only a person who knows to read the Haftorah properly is to be called up for Maftir. Nevertheless, many are accustomed to being lenient to call up even a person who cannot read the Haftorah properly, and have him say the blessing and have the Baal Korei read it aloud while he reads along to himself.
May someone who is blind, or illiterate, be called up for Maftir/Haftorah?
This follows the same law as a regular Aliyah, and thus practically in a time of need, such as Kavod Habriyos, he may receive the Aliyah and be Yotzei through hearing the Baal Korei read the Haftorah. However, he initially is not to be called up for the Maftir/Haftorah of the four Parshiyos, and Yom Tov.
 Admur 284:9; Rama 284:4; M”A 284:3; Ketzos Hashulchan 88:1; See Kaf Hachaim 284:19-22; Piskeiy Teshuvos 284:9
 1st and Stam opinion in Admur ibid “If a child was called up for Maftir and after he read from the Torah it was discovered that he cannot say the Haftorah, and the same applies if this occurred with an adult, then another individual may say it.”; Beis Yosef 282, brought in M”A ibid, that this may even initially be done and so is done in some communities to call up a child for Maftir and then have another read the Haftorah
 The reason: It is not necessary in such a case to have the person who will now read the Haftorah, receive an Aliyah, as the first reading of Maftir suffices. Now, although the Sages instituted that the person who says Maftir must first read from the Torah out of respect for the Torah so that the Torah be read before the prophets, as explained in 282:10, nevertheless, even when a child or another individual read Maftir from the Torah beforehand, the Torah has received its honor. The reason for this is because when a person goes up for Maftir it is evident to all that he is doing so in order so the Haftorah be read after the reading of the Torah, as certainly no one will say that he is reading it on his own behalf and not for the sake of the Haftorah, as the obligatory Aliyos have already been read, and Kaddish was recited after them. [Admur ibid; M”A ibid; Kaf Hachaim 284:21]
 2nd opinion in Admur ibid; Rivash 326; See M”A ibid
 P”M 284 A”A 3; Kaf Hachaim 284:22
 Admur ibid; Rama ibid; M”A ibid; Tosefes Shabbos 284:3; Emes Leyaakov Kerias Hamftir 8 p. 122; Birkeiy Yosef 284:6; Kaf Hachaim 284:19 and 23
Sephardic custom: Some Sephardic communities are accustomed like the first opinion to even initially call up for Maftir one who does not know to read the Haftorah. [Beis Yosef 282, brought in M”A ibid] Such communities are not to be protested. [Birkeiy Yosef ibid; Kaf Hachaim 284:23]
 Mateh Yehuda 284:5; Kaf Hachaim 284:19
 P”M 284 A”A 3; Kaf Hachaim 284:22
 M”B 284:8, brought in Ketzos Hashulchan 88 footnote 1
 Yaskil Avdi 7:14; Minchas Yitzchak 9:22; See P”M ibid, Kaf Hachaim 284:22, Piskeiy Teshuvos ibid
Opinion of Admur: On the one had Admur ibid makes no differentiation in this matter between a community in which they are accustomed to read the Haftorah along with the reader, and a community that does not. This is despite the fact that the custom of reading along is mentioned elsewhere in Admur. However, it is also unclear if the intent of Admur in the debate above is referring to a case where the Maftir does not know how to read at all, or even to a case that he can read it quietly to himself but cannot read aloud being that he does not know to read it correctly. Likewise, it is unclear if Admur only refers to a case where a second person is asked to read the Haftorah with its blessings, or even to a case where the Maftir will say the blessings and have the reader read it for him. Vetzaruch Iyun as why the Takana for Haftorah should be more severe than Kerias Hatorah, in which the custom today is for the Olah to read the blessings, read along silently, and have the Baal Korei read aloud.
 See Biur Halacha 282 “O Bedaled Parshiyos”; Minchas Yitzchak 3:12; Piskeiy Teshuvos 284:9