// specify amount for return
BigDecimal amountToRefund = new BigDecimal(10);
// specify amount currency
Currency currencyOfSale = Currency.getInstance("EUR");

// perform sale Return (Refund)
accept.sales()
    .saleReturn(Collections.singletonList(new CashTransactionForReturn(amountToRefund, cashRegister.getId())), originalSaleID, currencyOfSale, null, true, null)
    .subscribe(sale -> {
            // Returned Sale detail
        },
        throwable -> {
            // handle error
        }
    );