Is it permissible to invest a portion of one's personal financial trust without the company's knowledge, given the ability to repay the amount upon request?

The funds placed with you are trusts (amanat); it is not permissible to invest them for your personal benefit, as that constitutes a betrayal of the trust. Allah Almighty says: {O you who have believed, do not betray Allah and the Messenger or betray your trusts while you know [the consequence]} [Al-Anfal: 27]. The Prophet, peace and blessings be upon him, considered betrayal of trust to be one of the characteristics of hypocrites. This is not permissible except with the administration's permission.