I have a mobile home that is wired for a 90 amp furnace. but didn't have the furnace.. so I purchased one which is 60 amps. could I take the wires for the 90 amp furnace run it from the 90 amp breaker into the main lug in a subpanel and use a 60 amp breaker for this other furnace?
You can simply continue to use the 90 amp wires. You change the breaker to 60A, they're about $10, but hold on to the old 90A breaker because those critters are expensive.
This depends on the rule that you are always allowed to use larger wires than required.