Sforno on Deuteronomy 23:21/ספורנו על דברים כ״ג:כ״א

1לנכרי תשיך, you are allowed to pay interest to the gentile (on your loan from him) and you must not betray his trust by citing the prohibition to charge or collect interest.

2ולאחיך לא תשיך, even though according to the deal you made you obligated yourself to pay him interest and you are perfectly willing to pay same.

3למען יברכך, in order that you will receive G’d’s blessing instead. By not desecrating G’d’s name in your dealings with the gentile by reneging, you will merit G’d’s blessing.