How can I reconcile a partial amount of opening positive balance to an invoice for BP?

Hi all, 

I person a concern spouse for which determination was an opening equilibrium of 30 Euros. This is simply a outgo we had done for the supplier to support it connected relationship since we are ever purchasing tiny amounts from them. So fundamentally each invoice issued is of 2 Euros successful this lawsuit and we request to reconcile each clip by deducting 2Euros from the opening balance. 

When I effort to reconcile a partial magnitude of the OB to the invoice the equilibrium alternatively of showing 0 it shows arsenic -4. It keeps adding alternatively of deducting. If I effort to insert a antagonistic magnitude successful the OB magnitude to Reconcile it doesnt let me. 

Can idiosyncratic assistance maine please?