Respected Dr. Sahib, AOA,
It is true by 5:48 that the Rasool Allah was required to judge according to what Allah had revealed to him--even if what was revealed to him was different from what the POB had in their scriptures. However, we are told in 7:157 that Rasool Allah had come to liberate the POB from the yoke/shackles that they had on them because of the stricter and harsher legislation in their scripture. This was good news for POB and I trust that was the reason they would like their cases decided according to the new scripture (the Qur'an) because the punishments were expected to be much milder. For that reason, I believe there was no contradiction between the Qur'an and the earlier scriptures at the time of the revelation of the Qur'an. Additionally, the verse 5:43 informs us that Allah asked the Prophet why the POB came to him for adjudication of their cases when they had clear relevant guidance in their own scripture (Torah). This also supports the notion of convenience for the POB to use noble Prophet’s judgment, rather than the judgment of their own rabbis and doctors of law.