Month-End FX Controls in Google Sheets and Excel: Historical Pulls, Variance Checks, and Approval Tabs

What is verified in the current public product surface

The current public API surface is enough to build a finance-owned spreadsheet workflow without pretending a native spreadsheet add-on exists. Currency-Exchange.app documents rate lookup, conversion, historical date parameters, currency list/details, usage statistics, and usage export endpoints. That gives you the building blocks for workbook tabs, approval logs, and close-period review.

What is not clearly documented as a public native feature matters just as much. The current public spec does not publish a native Google Sheets add-on, native Excel connector, or native webhook-based spreadsheet refresh. This workflow is deliberately API-based: Apps Script, Power Query, Office Scripts, or a generic scheduler make the HTTP calls, and your workbook stores the control evidence.

Technical implementation: cURL, Apps Script, and Office Scripts

curl "https://api.currency-exchange.app/v1-list-currencies?active=true&pageSize=300" \
  -H "x-api-key: YOUR_API_KEY"

curl "https://api.currency-exchange.app/v1-get-currency-exchange-rate?from=USD&to=EUR&date=2026-03-31" \
  -H "x-api-key: YOUR_API_KEY"

curl "https://api.currency-exchange.app/v1-get-api-usage?from=2026-03-01&to=2026-03-31&service=currency" \
  -H "x-api-key: YOUR_API_KEY"

function refreshHistoricalRates() {
  const sheet = SpreadsheetApp.getActive().getSheetByName('Historical Rates');
  const apiKey = PropertiesService.getScriptProperties().getProperty('FX_API_KEY');
  const rows = sheet.getRange(2, 1, sheet.getLastRow() - 1, 3).getValues();

  rows.forEach((row, index) => {
    const [from, to, date] = row;
    if (!from || !to || !date) return;

    const url = 'https://api.currency-exchange.app/v1-get-currency-exchange-rate'
      + '?from=' + encodeURIComponent(from)
      + '&to=' + encodeURIComponent(to)
      + '&date=' + encodeURIComponent(date);

    const response = UrlFetchApp.fetch(url, {
      headers: { 'x-api-key': apiKey },
      muteHttpExceptions: true,
    });

    if (response.getResponseCode() !== 200) {
      sheet.getRange(index + 2, 7).setValue('ERROR ' + response.getResponseCode());
      return;
    }

    const data = JSON.parse(response.getContentText());
    sheet.getRange(index + 2, 4).setValue(data.exchangeRate);
    sheet.getRange(index + 2, 5).setValue(data.rateTime);
    sheet.getRange(index + 2, 6).setValue(new Date().toISOString());
    sheet.getRange(index + 2, 7).setValue('OK');
  });
}

type FxRow = {
  from: string;
  to: string;
  requestedDate: string;
};

async function main(workbook: ExcelScript.Workbook) {
  const sheet = workbook.getWorksheet('Historical Rates');
  const usedRange = sheet.getUsedRange();
  const values = usedRange.getValues();

  for (let row = 1; row < values.length; row += 1) {
    const [from, to, requestedDate] = values[row] as FxRow[];
    if (!from || !to || !requestedDate) {
      continue;
    }

    const url = new URL('https://api.currency-exchange.app/v1-get-currency-exchange-rate');
    url.searchParams.set('from', String(from));
    url.searchParams.set('to', String(to));
    url.searchParams.set('date', String(requestedDate));

    const response = await fetch(url, {
      headers: { 'x-api-key': 'YOUR_API_KEY' },
    });

    const data = await response.json() as {
      exchangeRate: number;
      rateTime: string;
      cached?: boolean;
    };

    sheet.getRangeByIndexes(row, 3, 1, 3).setValues([[
      data.exchangeRate,
      data.rateTime,
      new Date().toISOString(),
    ]]);
  }
}

type VarianceRow = {
  documentId: string;
  quoteRate: number;
  invoiceRate: number;
  settlementRate: number;
};

function buildVarianceRow(row: VarianceRow) {
  const quoteVsInvoice = row.invoiceRate - row.quoteRate;
  const invoiceVsSettlement = row.settlementRate - row.invoiceRate;

  return {
    documentId: row.documentId,
    quoteVsInvoice,
    invoiceVsSettlement,
    material: Math.abs(invoiceVsSettlement) >= 0.005,
  };
}

None of these examples assume a native spreadsheet integration. They are HTTP-based control patterns. That is exactly why they are durable: the same logic can move from Google Sheets to Excel, then into a no-code scheduler or warehouse later without changing the underlying rate policy.

Where this workbook fits in the broader workflow

The workbook is not the source of truth for every workflow. It is a control layer that helps finance review the rates already used in quotes, invoices, and payment flows. Use live conversion close to customer-facing events. Use historical dates for reporting and reconciliation. Use the workbook to make those decisions explicit and reviewable.

If the process grows beyond spreadsheet scale, the same structure maps cleanly into the warehouse patterns in the currency data warehouse guide and the policy model in the governance playbook. The workbook becomes your short-term control surface, not a dead end.