The Norse had many trade routes down the Volga as well as in the Mediterranean. They likely came in contact that way before being hired as mercenaries. Quite a few gravesites and hordes in northern Europe have silver coin and the like with markings in Arabic as well as Greek, etc.

Source: Oxford Press "Vikings," (title?)

Scandinavian traders -- mainly Swedes -- expanded throughout the Baltic Sea and began to sail down the great rivers of Eastern Europe (the Dnieper, the Volga, etc.) in the mid-8th century. Unlike the expansion towards the West during the same period, raiding and pillaging were not effective tactics in Eastern Europe. Though the area did feature expansive river systems, settlements in the region were much smaller, and the land itself was either densely forested or vast steppe regions. Therefore, the expansion of Scandinavians into Eastern Europe was focused on trade and settlement, rather than war (at first).

Thanks to their great sailing ability and trade contacts in the Baltic, Scandinavians quickly became major players in the areas where they settled. By the late 9th century, a mercantile elite of Scandinavian families had taken over the city of Kiev, where they ruled as princes and paid tribute to the Khazars. There was a north-south axis of significant Scandinavian influence established between Novgorod in the north and Kiev further south.

Contact with Constantinople was established early on, since the Byzantines would hardly fail to notice the political and commercial powerhouse, run by foreigners, being established in their backyard. Moreover, Prince Oleg was expanding the power of the Kievan Rus during this period, and even tried to sack Constantinople a few times between 907-915). Despite the aggression of Oleg, the Byzantines seemed to take favourably to these upstarts and ties increased during the 10th century. A trade agreement was signed at around this time that was mutually beneficial to both parties: the Byzantine Empire gained access to the lucrative Viking trade network that extended from the Mediterranean, through the Baltic, and to the North Atlantic. Meanwhile, the Scandinavian commercial elite in Kiev and Novgorod gained access to the Byzantine trade network, which gave them greater access to Arabic silver, Silk from the far east, and other luxury goods.

Further political integration would follow in the late 10th century. Vladimir I converted to Orthodox Christianity in 988, and married the sister of Emperor Basil II. It was at around this time that the Varangian Guard was established, a place for Scandinavian soldiers of fortune to make a name for themselves in service of the emperor (and brother-in-law to the most powerful Viking ruler in Eastern Europe).

Only for the first few decades was the Varangian Guard genuinely Scandinavian, though. Similarly, after the reign Vladimir's successor Iaroslav the Wise (died c. 1054), Scandinavian influence in Eastern Europe waned. Scandinavians were either assimilated into the local populations, or fled for greener pastures as the trade in Arabic silver collapsed and the power of Byzantium also declined.

I wish I had concrete sources, but this comes from the lectures in my Viking studies class I took last semester.