What is the ruling on an employee depositing temporary trust funds into his personal bank account to benefit from their interest?

It is not permissible to exploit a trust for personal purposes, as this constitutes a breach of trust, which Allah has forbidden and against which the Prophet, peace and blessings be upon him, has warned. This has been coupled with the sin of engaging in usurious transactions, for bank interest is indeed usury itself, which is forbidden by Islamic law.

One must repent to Allah for these two matters and dispose of usurious interest by spending it on general Muslim welfare, such as the poor and needy, and building hospitals.