There is a POST request that can be send for the generation of a credit invoice:

https://einvoice-demo.s1ecos.gr/Invoice/credit

In the Headers, APIKey must being included and as body the json below:

{
  "issuerTin": "EL094509431",
  "authenticationCode": "The authentication code of the issued document that you want to credit"
  "series": "The series of the credit that is going to be generated"
  "number": "The number of the credit invoice that is going to be generated"
  "internalId": "The InternalDocumentId of the credit invoice that is going to be generated
  "relateCredit": true // true: for 5.1 correlated credit invoice, false: for 5.2 uncorrelated credit invoice, it doesn't matter for 11.4 credit receipt
}

Important Remarks:

1. The request can be send several times for crediting the same 11.1 B2C retail receipt.
2. The request can be send only once for generating a 5.1 correlated invoice.
3. The request can be send several times for generating a 5.2 uncorrelated invoice.