There's a slang word in Russian, халява (pronounced halyava), meaning something that you get for free and without any effort despite its obvious monetary or effortful value. This something isn't either a present or reward; nor is it anything obtained illegally, say, by a theft or fraud.
Since some people (maybe too many) wouldn't mind getting something (even if they don't really need it) without paying for it, there appeared a saying "На халяву и уксус сладкий". Literally, it means somewhat like "Even vinegar, once you've gotten it for free, tastes sweet".
I'm wondering what would be the English slang or just colloquial equivalent of the Russian халява. "A steal", which came to my mind, suggests a bargain, so I don't think it matches the original meaning.
Also, I'd very much like to know if there are any English sayings or proverbs referring to getting something (unduly) for free and to a possible aftermath of such "lucky" events.
A small addition:
Also, I've been thinking of "on the house" idiom and wondering if "without the expense of buying", or "without any expense", or "gotten for free" could be acceptable.