Ruling on transgressing against the rights of kaafir workers

Firstly: 

One of the principles of good manners which is supported by
human nature and which Islam confirms is of great importance is rendering
back trusts and abhorrence of treachery and betrayal. Allah says
(interpretation of the meaning):

“Verily, Allaah commands that you should render back the
trusts to those, to whom they are due; and that when you judge between men,
you judge with justice. Verily, how excellent is the teaching which He (Allaah)
gives you! Truly, Allaah is Ever All‑Hearer, All‑Seer”

[al-Nisa’ 4:58]

“O you who believe! Betray not Allaah and His Messenger,
nor betray knowingly your Amaanaat (things entrusted to you, and all the
duties which Allaah has ordained for you)”

[al-Anfaal 6:27]. 

On this site we have previously explained that it is haraam
to take the wealth of kaafirs by deceitful means. See the answer to question
number 50716. We have also
explained the obligation of returning stolen wealth to its kaafir owners.
See the answer to question 47086
and also 7545,
14367 and
31234. 

Secondly: 

Whoever falls into wrongdoing, robbery or consuming people’s
wealth unlawfully then repents from that is obliged to return to each person
that which is rightfully his in this world 

It was narrated that Abu Hurayrah (may Allah be pleased with
him) said: The Messenger of Allah (blessings and peace of Allah be upon him)
said: “Whoever has wronged his brother with regard to his honour or anything
else, let him seek his forgiveness today, before there will be no dinar and
no dirham [i.e., the Day of Resurrection], when if he has any good deeds (hasanaat),
some of his good deeds will be taken and given to the one who was wronged,
commensurate with the wrong that he did, and if he  does not have any good
deeds, some of the bad deeds (sayi’aat) of the one who was wronged will be
taken and thrown onto him.” Narrated by al-Bukhaari (2269). 

And it was also narrated that Abu Hurayrah (may Allah be
pleased with him) said: The Messenger of Allah (blessings and peace of Allah
be upon him) said: “All rights will be restored on the Day of Resurrection,
until even the hornless sheep will settle its score with the one that has
horns.” Narrated by Muslim (4679). 

But if the one who is in possession of the property does not
know who the rightful owner is or where he is, or he is unable to get it to
him, then he should give it in charity on his behalf, then if he gets in
touch with him the owner of the property or wealth is to be given the choice
between having the reward of the charity going to him or having his wealth
returned to him and the reward for it going to the one who gave it in
charity. 

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him)
said: 

If he knows that his wealth includes permissible wealth and
haraam wealth but he does not know to whom it belongs, but he knows how much
it is, then he should divide the wealth according what is halaal and haraam
and take the halaal portion; the haraam portion should be given in charity
on behalf of its real owners, as should be done by one who possesses wealth
whose owners are unknown, such as money acquired by force, or things that
have been borrowed or entrust to him. The majority of scholars, such as
Maalik, Abu Haneefah, Ahmad ibn Hanbal and others said that he should give
it in charity. This is what is narrated in such cases from the Companions of
the Messenger of Allah (lessons and peace of Allah be upon him). If it is
not known how much is halaal and how much is haraam, then he should divide
the wealth in half, keeping half for himself and giving the other half to
its owners if he knows them, otherwise he should give it in charity. What he
gives in charity should be spent on the general interests of the Muslims: it
may be given to those who are entitled to zakaah, or spent on guests, or
used to help pilgrims go for Hajj, or be spent on jihad, or other charitable
causes that Allah and His Messenger love, as is done with all wealth whose
owners are unknown. This is what should be done by one who repents from
doing haraam things and has haraam wealth in his possession and does not
know who its owner is. End quote. Majmoo al-Fataawa (30/328) 

See also: Majmoo’ al-Fataawa 28/592 ff. 

Shaykh Mustafa al-Ruhaybaani (may Allah have mercy on him)
said: 

He has to give it in charity (when there is no trustworthy
ruler), as is the case nowadays. But if there is a trustworthy ruler, which
is more rare than red sulphur, then in that case he does not have to give it
in charity; rather he has the choice between giving it to him [the ruler],
so that he will no longer be responsible for it, or giving it in charity.
That is a sound view, provided that he takes responsibility for it before
the owners if he finds them, because giving it in charity without assuming
responsibility for it is a waste of the owner’s property. The is the ruling
concerning stolen property and the like, such as lost property that he
should not have picked up and he did no announce; in that case he may give
it in charity on behalf of its owner, provided that he will still take
responsibility for it before its owner.  

Mataalib Ooli al-Nuha, 4/66 

And Allah knows best.