I'm looking for a word that can describe a drink that someone bought you, for example, in return for doing a small favor for them. The word can be generic too, it does not have to refer only to drinks, it can refer to anything that someone else bought you.
For example, how could I make the following sentence shorter by replacing "bought by my friends" with a single adjective or noun?
Last Friday I spent a lot of money on drinks, even though many of them were (bought by my friends).
If there are no such adjectives/nouns in English, that's also a good answer to my question. I could not find any words with that meaning, but maybe I missed something.