What is the ruling on the mortgagor investing the mortgaged land and taking its produce for himself, what is the ruling on this produce, and what are the implications for the mortgagor and witnesses in this case?

The usufruct of the mortgagee from the pledge has details:

1. If the mortgagee benefits from the pledge without the permission of the mortgagor, this is not permissible, based on the saying of the Prophet, peace be upon him: "The property of a Muslim person is not lawful [to take] except with his willing consent." We do not know of any disagreement among the scholars regarding this.

2. If the mortgagor permits the mortgagee to benefit from the pledge:

If the debt is a loan, it is not permissible for the mortgagee to benefit from the pledge, even if the mortgagor permits it, because this is considered usury, based on the saying of the Prophet, peace be upon him: "Every loan that draws a benefit is usury." This is by the consensus of the scholars.

If the debt is not a loan (such as the price of a sold item, for example), and the mortgagor permits the mortgagee to benefit from it, there is no harm in that.