There is no such thing as a freewheel cassette. Most current 7 speed drivetrains have reverted to freewheels. For a short time when 7 speed was first introduced, there were 7 speed cassette drivetrains being produced. If your 7 speed freewheel is locking up, the most cost effective repair is to replace it unless you have almost unlimited free time and patience to repair it, and to be willing to swallow your pride when you fail and spend a very modest number of dollars for a replacement. If your bike was manufactured in the last 10 years, it almost certainly has a freewheel