Praise be to
Allaah.
If he does it before finding out that the one who wanted it
done said “Whoever does such and such will have such and such” does
not deserve the price named. But it is good manners to give him something
that will please him, that is similar to or more than the price for
this work.
From Fataawa Samaahat al-Shaykh Muhammad ibn Ibraaheem (may Allaah have mercy on him)