He collected money for his father’s medical treatment; what is the ruling on the surplus money?

Firstly: 

We commend you for your efforts with regard to your father’s
treatment, and we ask Allah to reward you for it and to bless your father
with a speedy recovery, and to cause what has befallen him to raise him in
status and erase his bad deeds. 

Secondly: 

With regard to the money that is surplus to the sick person’s
needs, reference should be made to the intention of the donor. If he
intended his donation to be charity to the sick person, then the money that
is surplus to medical costs is regarded as the property of the sick person,
and he may dispose of it however he wishes.

But if the donor intended that money to be for medical costs
only, or what he gave was the zakaah of his wealth, then in this case it is
not permissible for the sick person to take that extra money; rather he may
only take what is sufficient, and he must return the rest to the donor. 

Ad-Dardeer (may Allah have mercy on him) said: If a group of
people or one person helps him, and his needs were met. and there is some
money left over, or there was a shortfall, if they did not intend what they
gave him as charity, such as if they intended to ransom a slave, or they did
not have any particular intentions, then they may take back from that slave
whatever is surplus to his need. Otherwise, if they intended it as charity
to slaves who had contracts of manumission, then they cannot take back the
surplus money.

End quote from ash-Sharh al-Kabeer li’d-Dardeer. 

Circumstantial evidence may indicate the intention of the
donor. If he gave it after the patient’s situation was explained to him, or
in response to a request  for help for a certain purpose, this indicates
that his intention was to meet that need, and not to offer help in a general
sense. 

It says in Asna’l-Mataalib by Shaykh Zakariyya al-Ansaari
(may Allah have mercy on him) (2/479): 

If he gave him a dirham and told him: Buy yourself a head
cover with it, or go to the public baths, and the like, then it must be
spent for that purpose because that was the giver’s motive. That is if the
giver’s intention was to cover his head with the head cover or help him to
clean himself by giving him money to go to the public bath, because he saw
that the recipient’s head was bare, or that he was unkempt and dirty.
Otherwise, if he did not intend that and only said it by way of general
advice, then the donation was not given for that purpose, and he may keep it
and dispose of it however he wishes. End quote. 

Please see also the answer to question no.
126221 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was
asked: A man took some money from some kind people to use it to help him get
married, and there is some money left over. Should he return it to those who
gave it to him, or can he dispose of it in ways that will help him in his
religious and worldly affairs? 

He (may Allah have mercy on him) replied: 

The scholars (may Allah have mercy on them) say that
charitable gifts are permissible even for one who is well off. If those who
gave him money did so on the basis that it was charity, then he may dispose
of it however he wishes. But if what they gave him was zakaah that they gave
him for that particular purpose – namely to get married – then if there is
anything left over, he must return it to them, because he has no need of it.
But if he needs it for something else, such as furnishing his house, for
example, then he should ask permission from these people and tell them: I
paid the mahr and other costs of marriage, and there is some money left
over. I need some other things; will you allow me to spend it on those
things? If they say yes, then there is no problem, otherwise he must return
it to them. 

The basic principle we follow in this case is that whoever
takes money from people for a specific purpose should not spend it on
anything else until after he has asked their permission. 

End quote from al-Liqa’ ash-Shahri

If it becomes clear that the donors intended to give for a
specific need that you presented to them, then you do not have the right to
spend the extra money on any of your other needs, or on your family’s needs;
rather permission should be sought from the one who gave the money, if that
is possible. If it is not possible to find out who he is, or to get his
permission, then the money should be given for a purpose similar to that for
which it was given, as in the case of wealth that is established as a waqf
(Islamic endowment) for a specific purpose. 

Please see also the answer to question no.
114651

And Allah knows best.