You can't make a debtor payment from Customers 1st on an invoice in e-conomic.
Instead, you can make a credit note and then create a normal cash sale.
Here's how you do it:
Find the original e-conomic invoice sale in the sales history and click on it.
Click return sale
Click pay and pick the E-conomic invoice button. (That creates a credit note draft in e-conomic)
Now find the new return sale in the sales history (the one you just made)
Now click return sale again.
Notice that the sale now goes in correctly and hit pay, then pick your desired payment type.
Note: When you do this, the products will go back in stock on the credit sale in step 3 and then get pulled out again in step 6.