Yes, lymphoma/lymphosarcoma can cause hypercalcemia.  The abdominal mass is
likely a tumor.  In dogs, the higher the calcium the worse the prognosis.
I suspect the same is true for ferrets.  Yes there are other things such
as kidney failure that can elevate the calcium level, but that would have
shown up on the bloodwork.  Hyperparathyroidism is a rare possibility.
High dose pediapred is a good option, or you can consider chemotherapy.
 
Hope that helps,
Jerry
[Posted in FML issue 3429]