The Rambam brings this down in his discussion of the negative commandment of revenge :
In the words of the Sifra, "How far does the prohibition against taking revenge go? If a person requests, 'Lend me your sickle,' and the other won't lend it; and tomorrow [the latter] requests, 'Lend me your ax,' and he answers, 'I won't lend to you just as you didn't lend me your sickle.' In this case the Torah says, 'Do not take revenge.'"
But consider this scenario : two people A and B draft a business contract, A will deliver a shipment of axes to B by a certain date, and upon receipt B will deliver a shipment of sickles to A. When the deadline passes and A fails to deliver, B says "You didn't fulfill your part of the deal, so I won't fulfill mine".
So we have :
- "I won't lend to you just as you didn't lend me your sickle" - Forbidden because of revenge
- "I won't fulfill my part of the deal, just as you didn't fulfill yours" - (seemingly) Permitted because it's just a commonplace business dealing
But how and why are these two cases intrinsically different?