If one were to play professional basketball for a gentile owned team, where one of the requirements would be possibly practicing and playing on shabbos in a situation where nothing could be done to avoid it, would he be able to proceed if he is getting paying a yearly salary, and collecting payments monthly? What would be the difference between playing a game and practicing? And would he be required to return the proportional amounts where a practice or game fell on shabbos?

It is not appropriate for an adult to play basketball on Shabbat at all. (See Shmirat Shabbat Kehilchatah 16, 1). In addition he may transgress Shabbat prohibitions such as carrying out from one domain to another domain in a place that has no Eiruv (See Shulchan Aruch Orach Chaim 308, 45 and Mishnah Berurah 158) and other prohibitions. The Sechar Shabbos issue does not seem to apply here because he receives the Shabbat pay along with the salary for weekday. It is called Havla’ah which is permitted according to Halacha (See Shmirat Shabbat Kehilchatah 28, 58). In these types of queries, it is recommended to contact a rabbi who knows the people involved and is aware of all the details, so that he can guide the person how to avoid desecration of Shabbat.