Unlock My Phone

When I changed from T-Mobile, The Xfinity give me a Samsung A15 phone for free, the phone is locked to Xfinity, signed up for 24 months.  After 8 months, I changed a new phone (My own's unlocked phone Samsung Flip4), I want to unlock my A15 phone, so that I can give it to my friend.   The Xfinity said I must pay full for the A15 if I want to unlock it,  but the fact is I still stay in xfinity, and my monthly payment already include the free phone credits ($8.33 monthly credits).

 Is this reasonable?  can you help me with this issue?