Charging Interest for Non-Jews

I have seen it written that charging interest to non-Jews is an affirmative command. The verse cited is the one below. I'm am not seeing why that is a command to charge interest?

You may [however,] give interest to a gentile, but to your brother you shall not give interest, in order that the Lord, your God, shall bless you in every one of your endeavors on the land to which you are coming to possess.

It states [may] to a gentile because a gentile is not obligated to follow mitzvots such as not making money off your Jewish brother from their misfortune. If a Jew gives you their coat as collateral by the following day you must return it because it is a moral obligation. I can look it up, I read this not too long ago but I have been studying to where I can't remember anything. I need picture books!