patronize vs condescend

You are right in that both have a similar meaning, with a small difference; being condescending has a greater emphasis on the superiority of the person.

For example, a friend explaining the meaning of a word to you in a good-natured way, when you feel they should have assumed that you know the meaning yourself would make you feel patronised.

If on the other hand a friend explained the meaning of a word to you, and added "I wouldn't expect someone on your level to know that anyway", that would be incredibly condescending.

In your particular case, condescending would be closest but I wouldn't actually use either word. I would go for something like ungracious, impolite or rude.

Your friend's behavior is neither condescending nor patronizing. He's shamelessly claiming moral dividends on his good deed. He's treating you as if you were indebted to him for life.

Having performed one good deed does not automatically grant a person the right to be an asshole, but some people think it kind of does. What's even more intriguing is the fact that some others claim that right without performing a good deed first, and, guess what, you'll find lots and lots of them in key positions in every blessed field of human activity.