Praise be to Allah.
Allaah, may He be exalted, says (interpretation of the meaning):
“O you who believe! Eat not up your property among yourselves unjustly”
The Prophet (peace and blessings of Allaah be upon him) delivered a khutbah during his Farewell Pilgrimage on the Day of Sacrifice, and said: “Your blood, your wealth and your honour are sacred to one another, as sacred as this day of yours, in this month of yours, in this land of yours.” Narrated by al-Bukhaari (67) and Muslim (1679).
So you have to repent from consuming this unlawful money. Repentance means regretting it, giving up the sin, revolving not to go back to it, and restoring people’s rights to them.
Then you have to repay the amount that was discounted to the clinic; repentance cannot be achieved without doing that.
You do not have to tell the owner of the clinic or any of the employees about that, rather it is sufficient to give the money to them by any means. See the answer to question no. 31234.
As for not knowing how much it is, you can try to find out by counting how many times you went to the clinic and the amount of the discount, then add up the total discount that you received, until you think it most likely that you have given them their rights.
We ask Allaah to help you to do that which He loves and which pleases Him.
And Allaah knows best.