I once went to ask about mending a leather purse at one of those regular "arreglo de ropa" places, and she told me I would have to go to a shoe place. I would try those places first. They have a lot more experience with leather. I forget what they are called...something CALZADO, arreglo calzado. ¡No sé! They are everywhere and are small, little dark shops with the word calzado in the title somewhere.
I had a good experience with a pair of leather boots, which were basically leather with material on the other side and they took them in a bit - and on an angle! I would think a hem would be much easier. The boots ran me 30 pesos total! The place I went to was a block or two just south of Corrientes on Guardia Viejo in the Abasto area.