Warning: This question contains spoilers for Harry Potter 7.
"Hide them all, then," he croaked. "Keep them safe. Please." "And what will you give me in return, Severus?" "In in return?" Snape gaped at Dumbledore, and Harry expected him to protest, but after a long moment he said, "Anything."
Why would Dumbledore ask Snape for something in exchange for Dumbledore saving the Potters? Dumbledore also wants them alive, especially after hearing the prophecy. Was it a test to see if Snape really does love Lily and truly changed sides? If so, Snape could easily be lying, since he's a skilled legilimence.
Also: What did Dumbledore actually ask Snape to give in return?