In my regional language, there's a very good idiom which literally means…
'X' was already my favorite dish and a doctor advised me to be on 'X' (this 'X' diet/dish)
Now the context.
This idiom is used when someone wishes/likes something and on top of that, legally/officially he's asked to do that!
Example:
"Jack is not a dedicated person. He never likes to work."
"I know…but do you know he's been offered a field work."
"What? a field work? Oh my god! So now, he'll simply roam here and there!"
Yeah… [Idiom here] meaning he already love wasting time by not doing work and now he's offered a field work where no one will keep a watch on him. At least, in office, he was doing something as everybody watched.
To make it clearer –
It does not matter whether what you wanted/liked/doing is a bad thing or good. The idiom has nothing to do with goodness or badness. It's just what you already wanted/doing/liked, it's (by luck?) told/permitted/asked to do -all official and legal!
Best Answer
It's not common enough to be an idiom, but I've heard the expression, "getting paid to eat ice cream", though only in the first person and only concerning employment. So,
The origin may be a quote from musician Bob McQuillen, the biography of whom is titled "Paid to Eat Ice Cream", after something he said about being a professional musician.
You know, it sounds like your language has a very handy idiom there, for which there's no direct English correspondence. English being what it is, if you shared the literal translation, it might rapidly become an English idiom.