He wrote in the contract a price higher than that which was paid, and took it from the neighbour

This is not permissible. What you have to do is tell the
neighbour the true price; it is not permissible for you to deceive him, in
writing or otherwise. Allah has commanded the believers to be sincere to
one another and render back trusts. The Prophet (blessings and peace of
Allah be upon him) said concerning the two parties to a transaction: “If
they are honest and sincere, their (transaction) will be blessed, but if
they are dishonest and lie, the blessing will be erased from their
(transaction).” And the Prophet (blessings and peace of Allah be upon him)
said: “The believer is the brother of his fellow-believer, so he should
not lie to him or let him down or look down on him. Piety is here” — and
he pointed to his heart three times. And he (blessings and peace of Allah
be upon him) said: “None of you is (truly) a believer until he loves for
his brother what he loves for himself.” And Allaah (may He be glorified
and exalted) says: “Verily, Allaah commands that
you should render back the trusts to those to whom they are due” [al-Nisa’
4:58]. 

To sum up, if a person buys something for one thousand or two
thousand or whatever, then he records it with the judge or when registering
the sale as more than that, so that he can take more from the neighbour,
this is not permissible. End quote. 

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him)