I am currently handling a project. In the implementation phase of the project, they have 2 business units. the first BU is responsible for the technical requirements, the second is for the admin like billing, etc. From my company's perpective, we only see them as one and we will bill them as one but it looks like they have a conflict. How can I help them streamline their process? What is the best suggestion here?
If I am understanding your question and clarification correctly, your company will deliver a technical solution and then you will bill the client for the work. Your concern appears to be that the people within the client's company who will accept the technical part are not interested in the billing: they only want the technical solution. On the other hand, the people who will pay for the solution may disagree with their colleagues about the technical need (or the technical solution). Please correct me if I have misunderstood.
It seems that you anticipate having to act as the mediator between the two parts of your customer's business: this cannot be a good place to be, and you risk being criticised from both sides, or worse. In reality, if the client has internal issues, you can help to guide them, but it should not be your responsibility to solve them or to suffer as a result of them.
Without knowing the specifics, it is difficult to give specific advice, but in principle, I would recommend that the customer should nominate a single point of contact for the entire project. This person would then take ownership of the end-to-end communication between your client and your company (from the client's perspective), and your company should consider providing a similar single point of contact.
Handling the situation may require considerable tact and sensitivity. Your client may not like you suggesting how they should organise themselves, and you may be accused of overstepping your remit, so think about the approach and tread very carefully, even if they recognise the issues themselves. If you can sell the concept in a way that will benefit the client (as well as benefiting you), you are more likely to get a positive outcome.
This actually sounds like a pretty straight-forward Change Management issue. Your contract 'should' identify both who's responsible for acceptance of the deliverable, and who's responsible for payment (and those terms).
In the absence of that language, I would respond to your clients wishes about who to bill by sending a Request for Clarification to your client identifying the issue and asking for specific guidance. Explain that there seems to be some disagreement or ambiguity about who's responsible for what, and you need to clarify it before you get too far.
In the end, as a contractor, your primary job is to make sure you and your company get paid for supplying the project to the client (and in a timely manner). And you need to make sure that any issues that might hinder that are out of the way -- BEFORE you deliver the project.
A precision: you have to deal directly with the department of the technical requirements (I understood that your are doing something technical for them, you do something for them if you bill them?) Let that department contact their own billing department and settle their tension/conflict; ask them to settle it before you bill them. Ask for a name and address to send your bill. But no matter what, when you bill, you charge the hole compagny (with the compagny name on your bill); but their accounting/billing department will make you the payment of your bill.