An exchange of two things of equal value is a reasonable and honest trade.

The proverbial saying ‘fair exchange is no robbery’ is first found in a place we might expect to find it, that is, an early and comprehensive collection of English proverbs. There are a few of such but, in this case, it is John Heywood’s 1546 glossary A dialogue conteinyng the nomber in effect of all the prouerbes in the englishe tongue:

Though chaunge be no robbry for the changed case.