The fatwa in question is highly publicized, and is the basis that many so-called modernists allow interest-based loans. It may prove to be very difficult to find the actual text of the fatwa since it appears to be merely rumored by the Khedive (ruler of Egypt and Sudan at the time, who had major disagreements with Muhammad 'Abduh), but is nowhere else to be found.
The background behind this fatwa is that the Egyptian Post had started a service at the time where one was allowed to deposit a sum of money in what was called Savings Deposit Book (دفتر التوفير), where one gets a higher-than-market interest rate (e.g., at present, it is 10.25%). Had there been such a fatwa, it would have been referenced by the Egyptian Post in their documentation, or documented in Al-Manar (the publication used to document fatwas in Egypt at the time). I could not find the fatwa in either documentation. However, there was a mention of it in Al-Manar periodical.
In Al-Mannar, Vol. 19, Part 9, p. 527-9, issued on 22 Feb. 1917 (I am attaching the scan of the article), Muhammad Rashed Ridaa ('Abduh's main student, mentioned in the sentence following your quote in your source), was asked about this fatwa, and whether it contradicted the Qur'an and the Sunnah. Ridaa responded as follows:
Had there been an official fatwa from the Imam [Muhammad 'Abduh] on
the matter of the savings deposit book, it would have been documented
in his collection of fatwas in the Ministry of Justice, and should be
sought there. I have not seen a fatwa from him on this matter, but I
have heard him in a conversation mention the opposition of
the Khedive to his view.
— Muhammad Rashed Ridaa (quote)
In summary, the article claimed that:
The Egyptian government, through a royal decree, established a
savings deposit book in the Egyptian Post to give the poor some
means of saving their excess income.
About 3,000 out of those who used this service refused to take the
interest earned.
The government [Prime Minister] consulted 'Abduh on how to make the postal
savings system sharia-compliant. He gave some verbal recommendations
on what needed to be modified.
The Prime Minister informed the Khedive of the changes required. The
Khedive was furious that the Mufti [Muhammad 'Abduh] was willing
to allow a riba-based system to be used. [It is unclear what the
rationale is since the system was setup by a decree from the Khedive,
and he seemed to object to his own system].
The Khedive invited a group of Al-Azhar scholars to study the matter
and provide their recommendations based on jurisprudence (fiqh).
The article claimed that the Khedive did so to appear to be the protector of the religion in front of the public from 'Abduh's collusion with the government to allow riba.
Apart from this article, I found no other reference to this controversial fatwa.
Here is a translation of the full article. This is my own translation, so treat with care:
Interest from savings deposit
(Q. 11) from the undersigned in Egypt
In the name of Allah, the Most Gracious, the Most Merciful
Dear Respected Sir, may Allah protect you:
Peace be upon you, and Allah's mercy, and His blessings. We often hear from people about the legalization as permissible [halal] of depositing money in a savings funds in the [Egyptian] Post, and taking the interest, which is no doubt a form of forbidden [haram] usury [riba] by consensus of Muslims, without a difference of opinions.
When debated about this, they often invoke the official fatwa issued by the Imam [Muhammad 'Abduh]. Since we have not seen this fatwa, and do not know its rationale, and since you were the closest to the Imam and most knowledgeable about his opinions and fatwas, we are turning to you to explain to us the Imam's fatwa, and to clarify if it does not contradict the book [Qur'an] and Sunnah. Especially, since probate law offices decided to deposit minors' money in these funds based on this alleged fatwa as they claim, as they say.
May your diligence be satisfactory as always expected from you.
Written by Abu Al-Ashbal
May Allah pardon him with His favor
(A. 11) from Muhammad Rashed Ridaa
Had there been an official fatwa from the Imam [Muhammad 'Abduh] on the matter of the savings deposit book, it would have been documented in his collection of fatwas in the Ministry of Justice, and should be sought there. I have not seen a fatwa from him on this matter, but I have heard him in a conversation, summarized as follows, mention the opposition of the Khedive to his view:
The Government has established the postal savings fund by a decree of the Khedive (Royal Command). This was to give the poor a venue to save from their earnings what exceeds their expenses. Nearly three thousand among the poor who deposited their money in the postal savings fund did not accept to take the interest they deserved under said decree.
The Government asked me for a legitimate way to make this interest halal, so that the poor people using the system do not sin. I responded verbally that it was possible, provided that provisions of mudaraba organizations were accounted for.
The Prime Minister approached the Khedive to modify the Royal Decree accordingly to become Sharia-compliant. His Royal Highness showed his satisfaction. The Prime Minister then told him: "We have consulted the Mufti [Muhammad 'Abduh] about this matter [and this was his view]", the Khedive became furious and asked: "How can the Mufti allow riba? I must consult other scholars on this matter."
His Royal Highness assembled in the Koubbeh Palace a committee of scholars from Al-Azhar, and tasked them with legitimatizing the [Egyptian Post] savings fund to appear in front of the public as the defender of the religion, and the applier of Sharia law that stopped the Government's intent of forcing Muslims into a riba-based system with the collusion of the Mufti, had he [the Khedive] not remedied it.
The scholars laid out a project that their assembly presented to the Ministry of Finance. The Ministry of Finance then presented the project to me [Muhammad 'Abduh], and I found that they adopted the ratification that I had mentioned verbally to the Prime Minister.
This is what I heard from him [Muhammad 'Abduh], and I think he said that said scholars belonged to four or three schools of jurisprudence [madhhab], but I am not certain.
The form that the scholars established to ratify the savings fund does not count as a form of forbidden nasee' usury that existed prior to Islam. Imam Ahmad [ibn Hanbal] made it clear when asked about "riba without doubt", as did others, that it was taking extra money in exchange for a delay in payment [of the loan]. He [Ahmad ibn Hanbal] said: "When the time of settling a loan comes, one pays or a higher interest is imposed. If the borrower cannot settle a loan when due, the lender adds to the loan term and the borrower adds to the sum to be paid." Ibn Hajar said that such form of forbidden usury multiplied over months, and devastated households.
— NOTE: My own translation, so treat with care.
