Can you afford to pay your semester bill upfront? You are not required to use a payment plan; you can also pay Northeastern directly. Here are their payment instructions: Payment Methods
Their website says that they accept both wire transfers and credit cards if you make the payment directly to Northeastern.
If you want to go through Sallie Mae, it appears that the standard two options are via debit card and check written in USD. Did you ever talk to your home bank if they are able to issue you a check in USD? My German bank was happy to do that for a fee.