My professional service practice received a purchase order for my retainer. Do I just bill them?

[Anonymous]

I have a standard simple service agreement for my professional service practice. The new client I have will be a good one but is the most bureaucratic I've ever encountered.

My agreement says they have to pay a retainer fee of X dollars for me to start. They sent me a purchase order for the retainer fee (which is odd as I have nothing to give them yet for their "purchase." Do I just invoice them referencing the purchase order number, and hopefully get my retainer?

I will ask the people there I am working with. I've just never encountered a purchase order in my line of work and don't want to sound like a total n00b.