Praise be to Allah, and peace and blessings be upon the Messenger of Allah. To proceed:
There is no contradiction between the fact that the Paradise in which Allah settled Adam عليه السلام was the Paradise of Eternity and the fact that Iblis tempted him by saying, as Allah تعالى says:
﴿And he said: “Your Lord has only forbidden you from this tree lest you become angels or become among those who are immortal.”﴾
[Al-A'raf: 20].
This can be explained in several ways:
It is correct that the Paradise in which Allah settled Adam عليه السلام was the Paradise of Eternity, as indicated by texts from the Qur’an and Sunnah. The scholars have also mentioned that the early generations and the leading scholars agreed upon this.
However, as far as we know, there is no text proving that Adam عليه السلام knew, before Iblis whispered to him, that the Paradise in which he had been settled was the Paradise of Eternity from which there would be no expulsion.
Therefore, it does not necessarily follow that Iblis’s promise of immortality conflicted with Adam’s knowledge.
Adam عليه السلام may have known that this was the Paradise of Eternity, while this did not necessarily mean that he would never leave it for a period of time and then return to it afterward.
Indeed, this is what happened. Allah caused Adam عليه السلام to leave Paradise for the earth. Then, when Adam and his descendants enter Paradise in the Hereafter, they will have the eternal life in which there is neither death nor expulsion from it.
Allah تعالى says:
﴿So We said: “O Adam! Indeed, this is an enemy to you and to your wife, so let him not drive you both out of Paradise, lest you become miserable. Indeed, you have there that you will never go hungry nor be unclothed. And indeed, you will never thirst therein nor suffer from the sun’s heat.”﴾
[Taha: 117–119].
These verses explain the blessings that Adam عليه السلام enjoyed in Paradise and warn him about his enemy and the cause of his expulsion from it, but they do not contain an explicit promise that he would remain there forever without being expelled.
Adam عليه السلام may have known that it was the Paradise of Eternity, but that does not necessarily mean that he knew that merely being in it required eternal immortality.
This is how Iblis tempted him. He made him believe that eating from the tree was the means by which he could attain immortality and an everlasting kingdom.
Allah تعالى says:
﴿Then Satan whispered to him, saying: “O Adam! Shall I direct you to the tree of immortality and a kingdom that will never decay?”﴾
[Taha: 120].
Thus, Iblis whispered to him that this tree was the path to immortality. This was part of his deception and temptation of Adam عليه السلام.
This view is supported by the verse in which Allah تعالى says concerning Adam عليه السلام:
﴿And We had already made a covenant with Adam before, but he forgot, and We found in him no firm resolve.﴾
[Taha: 115].
Adam عليه السلام may have known what concerned the tree and what would result from violating the command, and then forgotten it when the temptation occurred; or Satan may have caused him to forget what had been entrusted to him.
Allah تعالى says:
﴿And We said: “O Adam, dwell, you and your wife, in Paradise, and eat from it freely wherever you wish, but do not approach...﴾
[The supplied text continues incompletely here.]
Faith in Paradise in the Hereafter.
As for Adam عليه السلام, his existence in Paradise occurred before the abode of responsibility and trial on earth. It had been decreed that he would descend to earth, and that he and his descendants would undergo the trials and tests that Allah had decreed.
Therefore, there is no contradiction between the fact that the Paradise in which Allah settled Adam was the Paradise of Eternity and the fact that Iblis whispered to him that if he ate from the tree, he would become one of those who are immortal. Iblis tempted him by making him believe that eating from the tree was the means of attaining immortality and remaining there, and he exploited the forgetfulness and temptation that occurred in order to cause him to disobey Allah’s command.
Then, after the end of the life of this world, the believers will enter Paradise in a way from which there will be no departure, and they will have eternal life without death.
And Allah تعالى knows best.