GST Invoice - negative Billing permissible ? Not credit note

Respected Sir,

As an exporter [ LCL Cargo] , we obtain a quotation from freight forwarder as follows:-

1. Ocean freight (-) USD 50

2. Origin changes [ all inclusive] - USD 125

Net payable --USD 75

Query is that in a cut throat competitive market, instead of reducing origin charges, they are giving a negative ocean freight. 

Do GST [ IGST] have such a provision to reflect a negative quote in Invoice? Having a doubt. Then as a service receiver, would we face any problem in future say IGST Credit.

Please advise.

Regards

Jayanta Bandyopadhyay 

Kolkata