Can he wipe over his socks if he put the right sock on before washing his left foot?

It is better and is more
on the safe side for the person doing wudoo’ not to put on his socks until
he has washed his left foot, because the Prophet (peace and blessings
of Allaah be upon him) said: “When one of you does wudoo’ and puts on his
khuffayn, he may wipe over them and pray in them, and he need not take them
off if he does not want to, except in the case of janaabah.” Narrated from
Anas (may Allaah be pleased with him) by al-Daaraqutni and by al-Haakim who
classed it as saheeh. 

It was narrated from Abu Bakrah al-Thaqafi (may Allaah be
pleased with him) that the Prophet (peace and blessings of Allaah be
upon him) granted a dispensation to the traveller for three days and three
nights, and to the non-traveller for one day and one night: if he purifies
himself and puts on his khuffayn (leather slippers or socks), he may wipe
over them. Narrated by al-Daaraqutni and classed as saheeh by Ibn
Khuzaymah. 

It is narrated in al-Saheehayn from al-Mugheerah ibn
Shu’bah (may Allaah be pleased with him) that he saw the Prophet (peace
and blessings of Allaah be upon him) doing wudoo’ and he wanted to take off
his khuffayn. The Prophet (peace and blessings of Allaah be upon him)
said to him: “Leave them, for I put them on when they (my feet) were in a
state of purity.” 

The apparent meaning of these three ahaadeeth and similar
reports is that it is not permissible for a Muslim to wipe over his khuffayn
except when he put them on in a state of full purity. The one who puts his
khuff or sock onto his right foot before washing his left foot is not in a
state of full purity. 

Some scholars are of the view that it is permissible to wipe
over the sock even if the person put his right foot into the khuff or sock
before washing his left foot, because in each case it is done after washing
the foot. 

But the first view is
more on the safe side, and it is more apparent based on the evidence. So the
person who has done that should take off the khuff or sock from his right
foot before wiping, and put the sock back on after washing the left foot, so
that he avoids an area of scholarly dispute and will be on the safe side
with regard to his religious commitment. End quote. 

Shaykh Ibn Baaz (may Allaah have mercy on him). 

Majmoo’ Fataawa Ibn Baaz,
10/116 

Another indication that it is not permissible to wipe over
the khuffayn unless one put them on when in a state of full purity is the
report narrated by Ibn Khuzaymah and al-Daaraqutni from Abu Bakrah (may
Allaah be pleased with him), according to which the Prophet (peace and
blessings of Allaah be upon him) granted a concession to the traveller for
three days and nights, and to the non-traveller for one day and night,
allowing them to wipe over the khuffayn if they put them on after purifying
themselves. Classed as saheeh by al-Khattaabi. Al-Bayhaqi narrated that
al-Shaafa’i classed it as saheeh and al-Nawawi classed it as hasan.
Talkhees al-Habeer, 1/278. 

See al-Majmoo’ by al-Nawawi, 1/541.