The most popular, versatile, widely-installed, easiest to use, and best-supported WordPress Business Directory plugin available.
The integration has to be done to each payment module you are using within BusinessDirectory plugin.
Follow the instructions for standard PayPal integration for ‘IPN used by other script’. You have to modify the file located in wp-content/plugins/business-directory-paypal/business-directory-paypal.php.
There is a function paypal_button() which contains the code of paypal button. And there is a function process_payment() where you have to place the cURL code.
Do not forget to integrate your WordPress with the click tracking code so the sale tracking works.