Intereses translates to "interest," as in the fee you pay on a loan. This word is the key to the song's clever central metaphor and title, "El Préstamo" (The Loan).
Maluma sings that he only lent his heart, he didn't give it away. He cheekily warns his lover, "Ojalá no olvides de pagar los intereses" (I hope you don't forget to pay the interest), implying his affection isn't free and requires her continued effort and attention.
Colombian reggaeton star Maluma treats romance like a high-stakes banking transaction in "El Préstamo" (The Loan). Right from the opening request to boost his mic, he makes it clear that someone is about to hear hard truths. He argues that he never gave his heart away forever: "Yo no lo di, yo no lo di, yo lo presté" ("I didn't give it, I didn't give it, I lent it"). Blaming bad experiences in the past where he lost too much in the "game of love," he wants his heart back so he can keep using it elsewhere: "No ves que lo tengo que usar / Contigo y con otras más" ("Don't you see that I have to use it / With you and with others too").
It is delightfully blunt, especially when he jokes that his partner shouldn't forget to pay back the interest ("pagar los intereses"). Instead of offering commitment, he advises her that there are plenty of men out there and suggests she find someone else to fall in love with: "Hombres hay de sobra / Por qué no te buscas quién te ame" ("There are plenty of men / Why don't you look for someone who loves you"). He even drops the classic proverb "Cada loco con lo suyo" ("To each their own," literally "each madman with their own thing") to brush off the drama and keep things moving.