Weird squealing noise when slowing down

[slow135i]

HI guys need some help. I took my car to bmw after I was experiencing a weird squealing noise when I was driving under 40 mph and slowing down (coasting under no load) once car stopped and idled it went away. Under any gas it went away. My initial thought was a vacuum leak, but then it sounded like a squeaky serpentine belt or pulley. The dealership has had my car for 3 days and called yesterday to say my rear main seal is leaking causing a vacuum leak. I dont believe that is possible as the rear main seal is a seal for oil in the motor correct? It shouldn't be under vacuum or boost as its for oil in engine. Help me here please they are trying to charge me $2500 for this and I cant believe its the actual problem.