Sifrei Devarim 263

¹(Devarim 23:21) "To the stranger shall you give interest, but to your brother shall you not give interest": "to the stranger shall you give interest" — a positive commandment; "but to your brother shall you not give interest — a negative commandment. ²R. Gamliel says: What is the intent of "but to your brother shall you not give interest"? It is already written ( Ibid . 20) "You shall not give interest to your brother" — There is antecedent interest and supervening interest. How so? If he thought to borrow from him and sent him (a gift), thinking "So that he lend me," this is antecedent interest. If he borrowed from him and returned his money and sent him (a gift), thinking "For his money that was 'idle' with me," this is supervening interest. ³"so that the L-rd your G-d will bless you in all to which you put your hands": Scripture "established" for him a blessing in all the sending forth of his hands. ⁴"on the land whither you are coming to inherit it": in reward for your coming, you will inherit.