What is the ruling of Islamic law regarding the position of the questioner whose employee borrowed a portion of a donation amount under his care, and then contact was lost with him, and is the questioner considered responsible for guaranteeing this money?

The employee's procrastination in repayment is impermissible if he is capable. Your consent for him to borrow from this money is also impermissible. Both of you must repent and guarantee the money. If the employee cannot be found, you are obligated to guarantee the money because your permission for him to borrow constitutes an تجاوز (transgression/overstepping) on your part concerning what you were entrusted with. Your hand over the money is a hand of trust, and it is not permissible to dispose of it except with the permission of its owners. You must hasten to repent and return the money. If you find the employee later, you may reclaim what you have paid.