Do you see any errors on the developer tools? You are linking the Value property to ${oferta.CodigoCliente}. What does it shows when you access that property?
Place a breakpoint right after you setting this Value property and post de value of oferta. I'd recommend initializing oferta as an empty array []. Remove the ValueProperty of the dropdown and set the Value property to oferta (not oferta.CodigoCliente). After this oferta should host all the properties of the dropdown selected value and you can access them all via ${oferta.Property}.