What can be done about the 20 day rehab being paid to the wrong Nursing Home?

2 rehabs were involved, medicare paid out of sequence.My Mom had to stay in a facility while awaiting swelling to go down in her broken leg before surgery. The facility was across state lines due to location. She spent most of her 20 days in this facility but had to be moved back across state line because of medicaid. She is now out of rehab after 60 days in last facility. !st facility said medicare sent 2nd facility 100% pay instead of 1st fac receiving it. Now 1st fac wants her to pay $1040 to them because they did not get their bill into medicare 1st. It is not my Mom's fault-we did everything by the book and kept up with the days to make sure she did not go over the days while in 1st facility. Does she have to pay for a mistake by the nh? What happens if she does not have the money to pay?