How Can I Initiate a Chargeback in Sandbox?

If you pass 888888 for amount to this endpoint, it’ll create a Dispute

curl https://finix.sandbox-payments-api.com/transfers \
 -H "Content-Type: application/vnd.json+api" \
 -u USxxxxxx:xxxxxx \
 -d '
 {
 "fee": xxx,
 "source": "PIxxxxxxxxxxx",
 "merchant_identity": "IDxxxxxxxxx",
 "tags": {
 "order_number": "21DFASJSAKAS"
 },
 "currency": "USD",
 "amount": 888888
 }'