You need to answer your manager, but don't have a clue, try these randomly:

I'll circle back to you
I will run the numbers on it
Let's go back to the drawing board
Let's touch base in a bit. Ping me
I don't have the bandwidth right now
It's on my radar
Let's put this on the back-burner
I have too many balls in the air right now
I have a lot on my plate
I'll get back by close of play tomorrow
I'll have to deep dive/drill down into this and get back
I have a hard stop at X hours
Let's park that for now
It's in the pipeline