A number of the functions of the Head of the Faith were shared – could be properly exercised by either the Universal House of Justice or the Guardian of the Cause. The Huququ'llah, or Right of God, is one of those.
The Huququ'llah, or Right of God, is offered through the Head of the Faith at any particular time. There was no specific provision made by Baha'u'llah that after Him the Right of God should be paid through `Abdu'l-Baha; He simply designated Abdu'l-Baha as the "marja'", an Arabic term meaning the Center to Whom all must turn, and the Right of God was paid through Him.
Abdu'l-Baha used this term, marja', in His Will and Testament to refer to both of His Successors, the twin Institutions of the Guardianship and the Universal House of Justice. Here is a passage in the Will, where Abdu'l-Baha states that the Guardian is the marja':
"... he is the sign of God, the chosen branch, the Guardian of the Cause of God, he unto whom all the Aghsan, the Afnan, the Hands of the Cause of God and His loved ones must turn." (Will and Testament, p. 11, italics added)And here is a passage in the Will, where Abdu'l-Baha states that the Universal House of Justice is the marja':
"By this House is meant the Universal House of Justice, that is, in all countries a secondary House of Justice must be instituted, and these secondary Houses of Justice must elect the members of the Universal one. Unto this body all things must be referred." (Will and Testament, p. 14, italics added)In a highly significant Tablet to Jinab-i-Natiq-i-Ardistani, dated 15 Dhi'l-Qa'dih 1338, (i.e. about one year before His Ascension) Abdu'l-Baha states that the Universal House of Justice is the marja'. Although the letter to Abdu'l-Baha from Jinab-i-Natiq-i-Ardistani has not survived, it appears that the Islamic Tradition to which Abdu'l-Baha refers is the one from the Prophet Muhammad concerning His Book and His Family, quoted in paragraph 222 of the Book of Certitude, and discussed here.
The transliteration from the Persian of the relevant portion of this Tablet is as follows, as you can see in the following paragraph, even if you cannot read Persian. "Baytu'l-'Adl" means House of Justice:
"Hadith-i-ki su'al nimudih budid ta'alluq bi kawr-i-Furqan dasht ta taklif-i-nas ma'lum gardad. Dar in Kawr-i- A'zam Baytu'l-'Adl Marja' Unchih ra ki Baytu'l-'Adl hukm farmayad taklif-i- nas hamun-ast ." (Provided by an individual believer, emphasis added)
The Universal House of Justice has approved the following translation of this portion of the Tablet of Abdu'l-Baha; the word "marja'" is translated as "the body unto which all must turn":
"The tradition concerning which thou didst inquire pertaineth to the Dispensation of the Qu’ran and expresseth that which was incumbent upon the people at that time. In this most great Dispensation, the House of Justice is the body unto which all must turn: whatsoever the House of Justice should ordain, the same is incumbent upon the people." (Emphasis added)Based upon the provision in a Tablet of Abdu'l-Baha that the Huququ'llah is to be paid to the marja', and His express statement in this Tablet that the Universal House of Justice is the marja', the Universal House of Justice determined that it was authorized to receive and administer the Right of God.
This is similar to the fact that throughout his 36 year ministry, Shoghi Effendi carried out many functions which the Guardianship shared with the Universal House of Justice, that the House of Justice would itself have performed if it had been in existence. For example, the Institution of the Hands of the Cause was originally established in the Will and Testament as a subsidiary institution under the Guardianship; and the local and national assemblies would be guided by the Universal House of Justice. In the absence of a Universal House of Justice, Shoghi Effendi infallibly guided the assemblies. In the absence of a living Guardian, certain of the functions of the Guardianship which are shared functions, are now carried out by the Universal House of Justice.