I believe you are looking for:
if you love something, set it free. If it comes back it is yours, if it doesn't, it was never meant to be.
Or the alternative ending that I prefer:
if it doesn't, hunt it down and kill it.
It isn't an idiom per-say, but it is the closest thing I can think of matching your request.
This is kind of the opposite, "one bird in the hand is better than two in the bush."
You can't pick the ripe fruit until you have dopped the blemished one from your hand.
One expression is to "throw your hat back in the ring"
Another is to "throw the small fish back"
This is not an idiom, but you're description reminded me of one of my favorite quotes. "Don't give up what you want most of all for what you want right now."
"When one door closes, another one opens"
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