gift horse

(idiomatic) An apparent gift, that has substantial associated costs.

Example: 1837, Washington Irving, The Adventures of Captain Bonneville,
  Having now, as he thought, balanced this little account of friendship, the captain was about to shift his saddle to this noble gift-horse when the affectionate patriarch plucked him by the sleeve, and introduced to him a whimpering, whining, leathern-skinned old squaw, that might have passed for an Egyptian mummy, without drying. "This," said he, "is my wife; she is a good wife--I love her very much.--She loves the horse--she loves him a great deal--she will cry very much at losing him.--I do not know how I shall comfort her--and that makes my heart very sore."
1996, Daniel Clement Dennett, Darwin's dangerous idea: evolution and the meanings of life, page 352:
  To human beings, on the other hand, each meme vehicle is a potential friend or foe, bearing a gift that will enhance our powers or a gift horse that will distract us, burden our memories, derange our judgment.
1990 Fall, Daniel Tyree McElrath, “Abusing the Privilege”, Ploughshares, volume 16, number 2/3, page 154:
  "Don't worry. You can keep them. You don't have to pay for them." / Well, a gift horse and all that. I took the sneakers.
2000, Michelle Spring, Running for Shelter, page 164:
  The woman on the telephone in the science-faculty office found me an address for Timothy Butler in no time at all. Full marks for information retrieval. Zero for security. If I had been consulted, I would have insisted that callers produce something more compelling than a vague interest in locating a student before his address could be handed over. Still, what it is they say about a gift horse?
2009, John Hart, The last child:
  You know what they say about a gift horse, man. This is free help. Don't take it for granted.