Hi,
Normally you would be looking a way to make the release automatically in the background by the workflow. A simple example: User X creates the document, the payment gets "locked" and the workflow starts. Now in the workflow before the approval step, you could have a condition check, which would check whether the document was created by a user with a specific authorization role (or a user who belongs to a certain organisation or whatever). If the condition was met, the workflow would execute the release as a background step and the workflow would end.
Now I don't know the details of your workflow and the process itself. Possibly there is a simpler or better way to do things. But in general I would be looking a way (BAPI, function, etc.) to do the releasing automatically. You could perhaps find the way just by taking a look into the workflow and/or the business object behind it (there could be a method "Release" etc.).
Regards,
Karri