U.S. airlines have mortgaged gates, flight paths and just about any asset they could find in the back of a hangar to weather the coronavirus crisis. Now, they are considering selling miles in bulk to their credit-card partners to raise cash.

United Airlines Holdings Inc. and Delta Air Lines Inc. and their respective credit-card partners, JPMorgan Chase & Co. and American Express Co., have discussed unloading miles ahead of schedule and for less than the lenders would ordinarily pay, according to people familiar with the...