Is it permissible for me to benefit from money a person has deposited with me without informing him, provided that I return the full amount upon his request?

If a person deposits something with you, you are not permitted to dispose of it except with their permission, and you must safeguard it in the manner such things are safeguarded. If you dispose of it without their permission, you must seek their forgiveness. If they do not forgive you, then you must give them the profit generated from their money or reach a settlement with them on what you both agree upon, for reconciliation is permissible among Muslims as long as it does not make lawful what is unlawful or make unlawful what is lawful.