InvoiceService.java

public Optional<Invoice> createClientInvoice(String clientId, LocalDate date) {
    List<Fee> fees = feeRepository.findByClientId(clientId);
    if (fees.isEmpty()) {
        return Optional.empty();
    }
    Optional<Contract> optionalContract = contractRepository.findByClientId(clientId);
    if (optionalContract.isEmpty()) {
        return Optional.empty();
    }
    Contract contract = optionalContract.get();
    String invoiceCurrency = contract.getInvoiceCurrency().toString();
    CurrencyConversion conversion = exchangeRateProvider.getCurrencyConversion(invoiceCurrency);
    MonetaryAmount total =
            fees.stream()
                    .map(fee -> conversion.apply(fee.getAmount()))
                    .reduce(Money.of(0, invoiceCurrency), MonetaryAmount::add);

    total = total.subtract(total.multiply(contract.getDiscountPercent()).divide(BigDecimal.valueOf(100)));
    total = total.with(Monetary.getDefaultRounding());
    Invoice invoice = createInvoice(clientId, date, total);
    return Optional.of(invoice);
}