Where does “размен” come from?

размен (Russian) comes from Russian разменя́ть, from Russian раз-, from Old East Slavic роз-, from Proto-Slavic orz- — dis-.

Ancestry of “размен”, step by step

Step	Language	Word	Meaning
1	Russian	разменя́ть	—
2	Russian	раз-	asunder, apart; dis-, di-, un-; enlarged action
3	Old East Slavic	роз-	—
4	Proto-Slavic	orz-	dis-

размен — Etymology, Origin & Word History | Etymology Explorer