He interrupted tawaaf in order to go and relieve himself, then he came back and completed it. Is his tawaaf valid?

Being in a state of purity is an essential condition of
tawaaf being valid, according to the majority of scholars. But they
differed with regard to the case where a person becomes in a state of
minor impurity during tawaaf, then he goes and does wudoo’: should he
complete the circuits or start tawaaf all over again? There are two
views: 

The Hanafis and Shaafa‘is are of the view that he may
complete his tawaaf, even if there was an interval in between, because doing
the circuits consecutively is not a condition required in tawaaf. 

The Maalikis and Hanbalis are of the view that he should
start tawaaf all over again, because minor impurity (when a person breaks
his wudoo’ during tawaaf) renders tawaaf invalid. So he has to start all
over again. The same ruling applies if the interruption between circuits is
lengthy, because doing the circuits consecutively is one of the conditions
of tawaaf being valid. 

See: al-Mawsoo‘ah al-Fiqhiyyah (29/131) 

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him)
said:

If a person becomes in a state of minor impurity during
tawaaf, his tawaaf is rendered invalid, as in the case of prayer. So he
should go and purify himself, then start tawaaf all over again. This is the
correct view. There is a difference of scholarly opinion concerning this
matter, but this is the correct view with regard to both tawaaf and prayer,
because the Prophet (blessings and peace of Allah be upon him) said: If one
of you breaks wind whilst praying, let him go and do wudoo’, and repeat his
prayer.” Narrated by Abu Dawood; classed as saheeh by Ibn Khuzaymah. And
tawaaf is of the same nature as prayer in general. 

End quote from Majmoo‘ Fataawa ash-Shaykh Ibn Baaz
(10/160). 

Shaykh Muhammad ibn ‘Uthaymeen (may Allah have mercy on him)
said: 

With regard to tawaaf and sa‘i, it is stipulated that the
circuits be done consecutively, and if there is a lengthy interruption
between them, the first circuits are rendered invalid, and the individual
must start tawaaf all over again. But if the interruption is not lengthy,
such as if he sits down for two or three minutes, then gets up again and
completes it, there is nothing wrong with that. 

End quote from Majmoo‘ Fataawa ash-Shaykh Ibn ‘Uthaymeen
(22/293) 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was
asked: 

A man did tawaaf al-ifaadah, and during the tawaaf he became
in a state of minor impurity, so he went and did wudoo’, then he came back
and completed his tawaaf without starting all over again, thinking that this
was a valid action. What must he do now? 

He replied:

With regard to the tawaaf in which he became in a state of
minor impurity, then he went and did wudoo’, if we say that being in a state
of purity is a condition of tawaaf being valid, then the tawaaf in which he
became in a state of minor impurity was rendered invalid, so basing the
latter part of it on the first part is not valid. Based on that, he is now
regarded as not having done tawaaf al-ifaadah. 

But if we say that wudoo’ is not a condition of tawaaf being
valid, then we should see whether his looking for water and doing wudoo’
took a long time. In that case, his tawaaf is not valid either, because
doing the circuits consecutively is a condition of tawaaf being valid. But
if he found water nearby, then did wudoo’ and came back quickly, then his
tawaaf is valid. 

End quote from Majmoo‘ Fataawa wa Rasaa’il al-‘Uthameen
(22/357) 

Usually going to the washrooms during Hajj season, when the
place is very crowded, then doing wudoo’, takes a long time, which
interrupts the continuity between the circuits, so it is not valid to
complete tawaaf based on the previous circuits. 

Based on that: 

If you have not repeated tawaaf al-ifaadah up till now, then
your Hajj is not complete and you have to go back to Makkah and do tawaaf
al-ifaadah, because tawaaf al-ifaadah is one of the pillars or essential
parts of Hajj that must be done – unless you did this based on a fatwa from
one of the scholars, or following a scholar who said that. In that case you
do not have to do anything further. 

For more information, please see fatwa no.
49012 

And Allah knows best.