PO first, contract second?

Hey savages, this is a first for me, so thought to check if any of you has had a case like this.

I was just informed by a prospect they will be proceeding with us as the vendor who won the RFI, and that they already registered a PR (Procurement Request) in their system, based on our quotation, which will soon be followed up by a PO.

So far, we`ve only signed their GTC (General Terms and Conditions) which was part of the RFI documentation, and haven`t started the legal/procurement back and forth on the contract yet.

Have you ever had a case like this, where you first get the PO and only then start working on the contracts?