Feudalism itself - being the exchange of land (enfeoffment) for (feudal) services - is mostly a Frankish (Germanic or 'indigenous') law system. This tradition is completely different from the Roman tradition (especially with regards to property law and obligation law) and was important throughout the Middle Ages and the early modern period, albeit with a lot of changes throughout time. Europe at that time had a lot of small principalities and kingdoms, all having their own feudal traditions which developed over time and differed from one another. This feudal law - in general - did not develop into a very big system except in England, where it laid the foundation for what we now call the Common Law.

The Roman Law tradition is based on one big codification of Roman Law: The Corpus Iuris Civilis made by the byzantine emperor Justinian the Great in 534 AD. During the 11th and 12th centuries, when trade became more important in Europe, rulers wanted to properly regulate trade. The feudal laws didn't help them with this problem, seeing that it is mostly based on a society in which trade is not important. Therefore, they turned to the Roman texts which were already being studied at early universities. From there, Roman Law once again took off and became a very important part in most European nations and cities.

The two systems coexisted during this period and were used in court throughout the later Middle Ages and early modern period. When proposed a certain question, the court would first look at local traditions based on feudal concepts and if they couldn't answer the question laid down before the court (which was often the case), they would turn to the Roman Law.

I hope this helps :)