Is it permissible to collect money from patients for the purchase of surgical supplies at a price exceeding their real value, and to use the excess amount for other purposes without the patient's knowledge or consent?

It is not the right of the physician to oblige the patient to pay a fee for their treatment in free governmental hospitals. If there is a shortage of medical supplies, the physician should inform the officials. If the officials see that providing supplies requires funds from patients, then it is obligatory to ask the patient only for what they need. It is not permissible to use what they paid for the treatment of others or for medical conferences without their consent.

What you did, collecting money from patients in the manner mentioned, is impermissible. You must repent to Allah by abandoning the sin, regretting it, and resolving not to return to it.

As for guaranteeing the funds whose owners you do not know and cannot reach, it appears that nothing is binding on you other than repentance, because the funds have already been spent on public interests and benefits.