Praise be to Allah.
The employee should approach his work with an attitude of trustworthiness, dignity and honesty, and should avoid haraam earnings, lying, cheating and deceiving.
He should do the work required of him in a proper manner, without cheating in his earnings or doing anything that will lead to regret in the Hereafter.
See the answer to question no. 4651.
Saying that the medicine mentioned is not available in pharmacies is a dubious statement, because the basic principle is that medicine is more available in pharmacies than in doctors’ clinics; the doctor may have free samples that he gets from (pharmaceutical) companies and the like.
Whatever the case, it seems to us, and we know from experience, that if you had looked more, especially in larger pharmacies, you would certainly have found the medicine that you were looking for or you would have found a suitable alternative to it. It does not seem to us that there is any concession concerning what you are asking about, especially as it is not a case of necessity. So it is not permissible to hasten to tell lies or resort to deceit and tricks when there is no case of necessity and no pressure to do so.
Even if we assume that there is no alternative for this medicine, and that it is only available from this doctor (which cannot be the case), either the doctor should sell it with an invoice, or you can talk to your superiors at work so that they can find out the facts about the matter.
If children have half of their medical costs covered, as you say, then it is not permissible for them to submit papers showing anything but the true facts. If on this occasion you did not pay any of the costs for this consultation, then you do not have the right to ask the administration at work for anything.
If you did pay for it, then you have the right to ask for what you are allowed to, according to the conditions stipulated by your father’s work, but that is on condition that the papers you submit are truthful and are not made up.
See also the answer to question no. 113700
And Allah knows best.