Here is one more thing for already-stressed retail landlords to worry about: tenants underreporting their physical-store sales by subtracting returns for items bought online from those figures.

Simon Property Group Chairman and CEO David Simon said during a conference call Friday that some tenants are doing just that, calling it a material issue.

"As a point of reference, our reported retail sales grew a strong 4% [during Q1]," Simon said. "We know there are a significant number of retailers who are underreporting their sales numbers because they are deducting returns of online sales that were not previously recorded as store sales."

That is not allowed under the REIT's leases with its retailers, and Simon said while "we plan to continue to provide reported retail sales, it is important to the investment community to understand that we believe this metric is understated."

Besides any damage to a mall or mall owner's reputation in the eyes of Wall Street, such a tactic on the part of retailers can reduce the amount paid to the landlord.

"We are getting dinged by internet returns," Simon said. "The only thing they are allowed to offset in terms of sales are returns from the store, and in many cases, we limit the amount of returns they are allowed to net against us."

Every retailer is different, and there is not a standard response yet, Simon said.

"How it gets dealt with is anybody's guess. It's certainly part of our lease negotiation. We just want to tell the market that we giving reported sales, and they are less than are going on, because of internet sales returns."

The issue comes at a time of headaches or worse for mall owners, and the expenses related to making their properties more competitive.

Simon Properties recently unveiled redevelopment plans for former Sears stores at five locations. The properties involved are Brea Mall (Brea, California), Burlington Mall (Burlington, Massachusetts), Midland Park Mall (Midland, Texas), Ocean County Mall (Toms River, New Jersey) and Ross Park Mall (Pittsburgh).

The redevelopments, most of which will get underway sometime this year, will feature enhancements such as new entertainment, fitness, restaurants and dining pavilions, residential, hotel and office space, as well as new retail brands.