I'd suggest not transferring wma to mp3 to attempt to master ..
if possible use the closest to the original wav file, even if it is wma.
Scott will likely be converting it back to wav to work with.

wav converted to wma converted to mp3 converted to wav and then mastered would likely show a few artifacts.

Just a suggestion on the process.
If you two need space for upload/download I can supply a sliver of storage ..

I do not work here, but the benefits are still awesome
Make your sound your own!