Are you receiving a blank page when clicking on "Receive Payment" in MyITCRM? I was able to fix this problem by uploading a revised include.php file to my webserver. You will need to download this zipped include.php file.
Rename your previous /modules/billing/include.php file to include.php.bak or whatever is conveniant for you. Then unzip and upload that new include.php file to /modules/billing/ directory, with your ftp program. Once uploaded now try to click on "Receive Payment" in MyITCRM. You should no longer receive a blank page.
Also, you can copy and paste this Fixes #268 - Blank page error on processing payment for invoice code from MyITCRM's Github site, using vi, gedit, or your preferred command line editor. Once on that page click on "View". Now try to click on "Receive Payment" in MyITCRM.
You should no longer receive a blank page and be able to enter in a payment for one of your customers.