I have a similar issue where a Commercial tenant defaulted on a lease.  The remaining deferred unamortized lease commissions MUST be written off.  Unamortized loan costs are just another category of Intangible assets on the balance sheet (and depr. sch.)  Here's a case reference related to leases, you might start there and then find similar IRS guidance to support W/O remain deferred unamort loan costs.  Then get back to Lacerte on your issue.  See Oliver Iron Mining Co. v. Com, 13 T.C. 416, 418, n. (1949).