Offering to contract, regardless of what the contract entails (excluding contracting someone to engage in theft), is moral as long as the person has the ability to refuse. If no money were offered and the girl (who I'm assuming is of age) were simply asked for sex it would not be immoral so why would the addition of a monetary incentive make it become so? It would be immoral if one were to forcibly get her hooked on heroin in order to take advantage of the vulnerability brought on by addiction. In that case the stolen property is her mental faculty. I see nothing debatable about the morality of the first example unless the girl was incapable of giving informed consent. I will be discussing consent in an upcoming post.
I am of the opinion that morality itself is not particularly complex but I certainly appreciate the difficulty in discovering all accurate information available in order to recognize whether theft has occurred or not. I do believe that trials might be more clear cut if jurors had a thorough understanding of the objectivity of morality instead of only being equipped with a subjective sense for it. Without solid indicators of what to look for when passing judgement they are more likely to be swayed by the rhetoric of the defense or prosecution. As you said, "Sometimes it takes a lot of work to uncover the real truth..."
This post was just a short outline of the topic and I will be discussing it in more detail in the future. Thanks for your thoughts!