def create_offer_on_existing_asin(sku, asin, price, quantity, fulfillment_channel):    
    # API endpoint and URL
    endpoint = "https://sellingpartnerapi-eu.amazon.com"
    request_url = f"{endpoint}/listings/2021-08-01/items/{seller_id}/{sku}"
    
    # Headers
    headers = {
        "x-amz-access-token": access_token,
        "Content-Type": "application/json"
    }
    
    # Query parameters (marketplace ID in query string)
    params = {
        "marketplaceIds": marketplace_id,
        "includedData": "issues"  # Optional, can include other sets
    }
    
    # Payload for creating/updating an offer on an existing ASIN
    payload = {
        "productType": "PRODUCT",
        "requirements": "LISTING_OFFER_ONLY",
        "attributes": {
            "merchant_suggested_asin": [
                {
                    "value": asin,
                    "marketplace_id": marketplace_id
                }
            ],
            "condition_type": [
                {
                    "value": "new_new",
                    "marketplace_id": marketplace_id
                }
            ],
            "fulfillment_availability": [
                {
                    "fulfillment_channel_code": fulfillment_channel,  # Use the correct fulfillment channel
                    "quantity": quantity
                }
            ],

            "purchasable_offer": [
                {
                    "currency": "GBP",
                    "marketplace_id": marketplace_id,
                    "our_price": [
                        {
                            "schedule": [
                                {
                                    "value_with_tax": price
                                }
                            ]
                        }
                    ]
                }
            ]
        }
    }
    
    # Send the PUT request to create/update the offer
    response = requests.put(request_url, headers=headers, params=params, data=json.dumps(payload))
    
    return response