How can a Saudi Visa debt be paid when its party cannot be reached, and is it permissible to disburse a portion of the money monthly with the intention of repayment, and how is the monthly installment amount determined, given the existence of an undisposed inheritance?

Whoever repents with sincere repentance, Allah accepts his repentance and erases his transgression, and it is not permissible to despair of Allah's mercy. Sincere repentance is possible. If you still have the visa in your possession, refrain from using it, and pay off its debt whenever you are able. Giving charity does not absolve one of debt as long as it is possible to deliver it to the rightful claimant. Pay off what you can of the debt monthly, and there is no blame in delaying what you cannot. With a sincere intention, Allah will help you pay off the debt.