hi,
so, basically you need two pr of 200 + 100, instead of 300.
you can achive this by converting PR to PO, what ever qty you required.
In that case you can convert PR to PO of 200 qty and then remaining 100 qty.
in my opinion there is no standard solution available, you can customize it like when you need to create PO it will check the last confirmed qty. but still it is not good solution in my opinion.
or you can give a pop up message when converting a PO would be best idea.
regards,
Solomon