Total Posts: 812
34,mantri Sadan,Shivaji Nagar,Nagpur,MS,India 4400
"Unable to Save Invoice” if magento is echoing this then you need to debug/analyze.
For that you need to check error report or exception.
So switch on these from admin and then in var folder you will have those logs to look further.
post further details(logs) to comment further.
When I try to convert and order to an invoice in the admin, I get the following error message: “Unable to Save Invoice”. Magento ver 1.5.1