I have been working on a software project for a client for several months. Up until now, there have been few issues involving payment. Now that the project has come to an end, the client is refusing to pay the last of my invoices without a W9. It is my understanding that the W9 is filled out after the payment has been received. I have let the client know that I will gladly provide them with the form after receiving payment for services rendered. Am I misunderstanding how the form works? Isn't the form only necessary when filing taxes? What are my available options?
According to the IRS site
Use Form W-9 to provide your correct TIN to the person who is required to file an information return with the IRS to report, for example, ... income paid to you ...
Since the client is already obliged to file an information return disclosing the payments already made, you should provide the W9 without delay. You would be obliged to provide your TIN even if your final invoice was never paid.
In any case, it costs you nothing to furnish this information when asked. If it is more convenient for the client's accounting to provide the W9 now, why not do so? Arguing over this point will not make you remembered more fondly when the client has another project.
It may be that the client has encountered or fears to encounter a contractor who was paid and then never furnished a W9, leaving the client to have to explain why the proper 1099 was not filed with the government.