Praise be to Allaah.
The general principle with regard to material damage is
that the one who damages something is liable for it, whether he did it
deliberately or by mistake, but there is no sin on one who did it by
A hired worker -- such as the plumber -- is not liable unless
he overstepped the mark or was negligent. What you mentioned about him
forgetting to close the main water line is regarded as negligence and
carelessness, so he is liable for whatever was damaged as a result of that.
Al-‘Izz ibn ‘Abd al-Salaam (may Allah have mercy on him)
said: Liability dictates compensation, and compensation is not waived in
cases of forgetfulness. End quote.
Qawaa‘id al-Ahkaam, 2/4
Al-Zarkashi (may Allah have mercy on him) said: Forgetfulness
means that there is no sin in cases of damage, but it does not mean that the
person is not liable. End quote.
In al-Mawsoo‘ah al-Fiqhiyyah (7/167) it says: If a
hired worker is neglectful, then he is liable. End quote.
Based on that, the plumber is the one who is liable for the
damage caused to the laptop. The basic principle with regard to liability is
that what can be replaced must be replaced, and if the damaged item cannot
be replaced, its value must be paid instead. So he has to buy something to
replace the damaged part, if possible, otherwise he has to pay its value.
The amount in question should be determined in consultation with experts.
And Allah knows best.