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It doesn't exactly work for me, but I guess it conveys the fundamental meaning.

Or, maybe...

Roses are red, violets are blue
And he's not coming back to you!

:)

So, if I may use a little artistic leeway here to get across the gist of this saying -

"This I know and know it true: If it's not yours, it won't come back to you."

Short and sweet?

So...

"I know the simple truth, and I repeat only that: What is our own does not escape us. What goes away is not yours."

Clearer, but still confusing.

OK, here is another one of those Russian inspirational quotes which I am having difficulty translating. I understand the words, but their meaning here escapes me:
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