Where the hell did you get something like that?! Rinoske left off the verb so we're left "guessing". (Could be, for example, "find" instead.) Also, I'm not aware of a way to make this style of unfinished sentence work in English, so I translated it to a finished sentence and added "!" because he seems to be shouting/surprised.

So aliens really exist...!

It fell down from the sky (lie)

With this...

Couldn't we make a fair exchange?

But!

This is a treasure to me too, so I will only leave it in your care temporarily. "...so I will only temporarily leave it in your care" would probably be a bit more precise, and allow a more literal translation of the next bubble. On the other hand, I think it sounds a lot more awkward, and I think this gets the point across well enough.

O... only... temporarily? 預ける is technically the "leave in your care" from the previous bubble, but the point is that he doesn't get to keep it.

In that time, you can do whatever you want with it to your heart's content

I will let you borrow it for a short while!