Transferring prepaid credit


I mistakenly bought a prepaid plan for my wifi modem which has both data and dollar credit, a data pass plan. I can't use the $credit because I can't make calls from my computer.

If I change to a data only plan I'll probably lose the credit I have built up over the past year or so (around $300 now).  How can I either use this credit or transfer it to my own prepaid mobile or at least to a friend's?