Tiina63, I normally devote my posts to matters that warrant a Christian perspective, but your situation calls out to me at this time. I don't have a cat, but I like them, along with all pets and animals. Allow me to give you a different perspective.
I understand you love your cat and would prefer to keep him if the expenses are manageable. This is the case for most people, though the decision of acceptable management costs certainly varies. There are 2 points I think you should consider here.
First, you, as a human being made in the image of God, with a conscience and eternal soul, are far more valuable than any pet or animal - regardless of how much that pet is loved. And thus, your physical and emotional well being, along with your need to interact well with other people, far outweighs the needs of your cat.
Second, almost everybody has a breaking point on the amount of money they're willing or able to spend on their pets. Unlike humans, this just seems to be an accepted and reasonable reality. And thus, pets are routinely put to sleep sooner or later, once their condition or expense warrants. In your case, you have the unique privilage of avoiding this heartache process, thanks to your option of returning the cat to its previous owner. (Admittedly, you don't know what care he'll receive afterwards, but it may be comparable to what you're able to provide yourself; you really don't have the foresight to factor this concern into your deision making process.)
So, only you can decide whether to hold onto your cat or return it to your ex-friend. But, may I suggest that, in this case, you can do what's best for you without feeling guilty. I'm confident that your concern and negative impacts over this matter are more relevant and important than any feelings or impacts that the cat will ultimately realize. You must decide whether living without the cat will be too much of a loss that negatively affects your life, or if freeing yourself from the responsibility will give you your healthy life back. So, keep guilt out of the decision making process and keep your priorities right. Your human life is more important than any animal life.