DETROIT -

An investor group is offering bankrupt Detroit a $4 billion loan in exchange for the city offering up the Detroit Institute of Arts's collection as collateral.

It's a bombshell offer coming at the 11th hour of the city's bankruptcy case, and it's one that Emergency Manager Kevyn Orr said is going nowhere.

"This is an attempt -- nothing more -- from our creditors to get more money from them, to improve their recovery at the expense of retirees and at the expense of selling the treasured assets of the city," said Orr spokesman Bill Nowling.

Orr and his staff say the $4 billion loan-for-art proposition isn't workable for many reasons, primarily because of the all-out, gloves-off, knock-down fight it would create over what art and whether any of it can be sold.

Complete coverage: Detroit bankruptcy

The offer would blow up the carefully negotiated "grand bargain" which puts up more than $800 million in state and private investment to backfill Detroit retiree pensions and safeguard the collection.

"Look, let's just get one thing straight: the city is fully 100 percent committed to the grand bargain and it's going to defend that in court. That's the best plan on the table that's going to protect pensions. It's going to preserve the art and it's going to allow the city to move forward out of bankruptcy," said Nowling.