Hi Marta, thanks for joining this thread with such detailed information. You won't need to enter an open bill to record the supplier refund. 

Please check that the bank deposit has the default account for Creditors (or Accounts Payable) selected in the Account field, with the supplier selected under Received from. 

When linking to supplier credit and bank deposit create a Cheque (via + New transaction on the supplier page). The panel on the right will then allow you to add the two transactions. 

If you're still unable to complete this, please contact our chat support. They'll have the tools available to set up a screen share to view this with you.