Always remember that you should not make any modifications in BDM.
If you are making some changes(very little)-you should provide reasonable justifications. One should not change the problem altogether.
is it maybe "Utility International" assignment? I have similar problem, and those relations don't make too much sense for me. I believe it should be:
Customer 1 - * Bill
Customer 1 - * Payment
Payment 1 - * Bill
I have seen this problem in real projects many times that business analysts come up with doamin model and they are to be treated always as analysis level (i.e. logical domain model) which explain business requirements.
But solution/design level domain model (physical doamin model) can have subtle differences and not necessarily be identical to logical one.
Correct me if I am wrong. What do other chaps think?