Is the hundred pounds gained from selling an antique (vase) and informing its owner of a price other than its actual price lawful or unlawful?

The hundred (currency unit) is not permissible for the questioner unless he informs the owner of the true price and the owner consents to it. This is because it is not permissible for an agent to take anything for himself without the knowledge of his principal. The Sharia has forbidden taking a Muslim's money without his willing consent, and it has forbidden lying and betrayal of trust. Therefore, the questioner must repent and seek forgiveness, and return the money to its owner or seek his halal (permission/absolution).