Hi If you are using paypal standard you can try below code which convert your amount fron INR to USD.
File path:
\app\code\core\Mage\Paypal\Model\Standard.php
In above file you need to change in getStandardCheckoutFormFields function.
Note:Please do not change in core files.copy it from core or rewrite the
model
place the below code after $result = $api->getStandardCheckoutRequest(); line.
<?php
$baseCode = Mage::app()->getBaseCurrencyCode();
$fromCur = Mage::app()->getStore()->getCurrentCurrencyCode();
$toCur = 'USD';
$allowedCurrencies = Mage::getModel('directory/currency')->getConfigAllowCurrencies();
$rates = Mage::getModel('directory/currency')->getCurrencyRates($baseCode, array_values($allowedCurrencies));
$result['amount'] = round((($order->getGrandTotal() * $rates[$toCur])/$rates[$fromCur]),2);
$result['currency_code'] = $toCur;
$j = 0;
$items = $order->getAllItems();
foreach ($items as $itemId => $item)
{
if ($item->getParentItem()) {
continue;
}
$j ++;
//$result['amount_'.$j] = round(Mage::helper('directory')->currencyConvert($item->getPrice(), $fromCur, $toCur), 2);
$result['amount_'.$j] = round((($item->getPrice() * $rates[$toCur])/$rates[$fromCur]),2);
}
$j++;
$result['country'] = $order->getBillingAddress()->getCountryId();
$shippingSporter = $order->getBaseShippingAmount();
//$result['shipping'] = round(Mage::helper('directory')->currencyConvert($shippingSporter, $fromCur, $toCur), 2);
$result['shipping'] = round((($shippingSporter * $rates[$toCur])/$rates[$fromCur]),2);
//$result['discount_amount'] = -1*round(Mage::helper('directory')->currencyConvert($order->getDiscountAmount(), $fromCur, $toCur), 2);
$result['discount_amount'] = -1*round((($order->getDiscountAmount() * $rates[$toCur])/$rates[$fromCur]),2);
$result['discount_amount_cart'] = $result['discount_amount'];
$result['amount_'.$j] = $result['shipping'];
unset($result['discount_amount']);
unset($result['shipping']);
unset($result['discount_amount_cart']);
unset($result['amount_'.$j]);
return $result;
?>