Selling home while under contract with property management

[Ben M.]

I am selling my rental property and wanted to use my realtor but the property management is telling me that they will no longer be able to manage my property if I use my realtor. They will only continue to manage only  if I use their realtor. They said something regarding liability reasons. I didn’t see anywhere in the contract regarding this. Is this common practice for them to stop managing my property if I use my own realtor to sell?