Silver coins from the Islamic world were not exactly rare in Scandinavia (e.g. https://www.medievalists.net/2023/11/nearly-500000-dirhams-w...), and trade is a much more likely explanation especially over such large distances. AFAIK trading vs raiding was often just a question of opportunity and might have been done by the same viking/trader group depending on what was more 'convenient' (or: first raid and then setup an adhoc trading point just around the corner to exchange the loot against silver coins because those are easier to carry home - or the classic: do slave raids on the way from Scandinavia to the Black Sea, and trade the captured slaves with the Greeks-slash-East-Romans and Muslims against silver).