Praise be to Allaah.
The arrival of this money in your account may have been a
mistake or it may have been deliberate. If it was deliberate -- which is
unlikely -- then it is money that the owner did not want and he gave it to
you, so you are entitled to take it. If it was a mistake, which is what
seems to be the case, then it must be returned to its owner. If it is not
possible to find out who the owner is, then you should give it in charity on
his behalf, on the basis that if you find out someday who the owner is, you
will give him the choice between approving of that charity or taking the
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him)
If he has money and does not know who its owner is, such as
one who seized it by force and has repented, or one who betrayed another and
has repented, or one who dealt in riba and has repented, and so on, who now
have money in their possession which does not belong to them and do not know
who it belongs to, then he may dispose of it by giving it to the needy and
spending it in the interests of Muslims. Al-Fataawa al-Kubra
And he (may Allaah have mercy on him) was asked about
pilgrims who encountered some Bedouin who had raided some people on the road
and taken their fabric; the bandits fled and left behind their camels and
the fabric. Is it permissible to take the camels that belong to the thieves
and the fabric that they stole, or not?
Praise be to Allaah. Whatever they took of people’s wealth
must be returned to them if possible. This is like lost property which
should be announced for the year. If its owner comes, all well and good,
otherwise the finder may spend it on the condition that he guarantee (to
replace it or compensate the owner if he shows up).
If there is no hope of finding the owner, then he should give
it in charity and spend it on the interests of the Muslims.
The same applies to any property whose owner is not known,
such as property that was seized by force, borrowed items, items left in
trust with another, and stolen wealth seized from thieves or lost property
that is found. All of this should be given in charity or spent on the
interests of the Muslims. Majmoo’ al-Fataawa (30/413)
And Allaah knows best.