I found this aphorism in my book of Korean aphorisms.
내 칼도 남의 칼집에 들면 찾기 어렵다 nae kal.do nam.ui kal.jip.e deul.myeon chaj.gi eo.ryeop.da my sword-TOO other-person-GEN sheath-IN fall-IF find-INF hard-PRES If my sword ends up in another's sheath, it's hard to find.
This has the same meaning as English’s “Possession is nine-tenths of the law.”