After being hit by first wave at Miller's planet in Interstellar, Cooper and Amelia discuss something about time slippage etc.
At that time Cooper says that, "can't we just jump in black hole to gain back all the years"?
Question is, how does Cooper know that jumping in black hole can return back him years or made time travel possible to him?
Cooper is basically in grief at this point and is asking (despite the fact that he knows the answer) whether the effects of the black hole can be used to reverse the passage of time as well as increasing it. It's then explained to him, as well as to us, the audience, that things don't work like that in the real world.
The novelisation makes his thought process a little clearer:
“Do you have some way we can jump into a black hole and get back the years?” he finally asked.
She dismissed that with a wag of her head.
“Don’t just shake your head at me!” he snapped.
“Time is relative,” Brand said. “It can stretch and squeeze—but it can’t run backward. The only thing that can move across the dimensions like time is gravity.”
He knew that. He’d read it. But he wasn’t ready to give up. Brand didn’t know everything—that much was abundantly clear.
Obviously he later discovers that time travel is possible (at least using gravity waves when suspended in a 11-dimensional construct masquerading as a black hole, controlled by alien humans from a distant future timeline) but that's something he couldn't possibly know at this stage.
