Correct/edit PHP file Joomla/Virtuemart Payment Module

Dear Friends, Some times age we have developed payment module plugin for Joomla/Virtuemart "PayGe" from Libery Bank. The plugin is installed on site and works properly. The same bank has also installment service and we will to implement it in Virtuemart too as another payment plugin. As I did understand from documentation, "installment" works almost in the same way, as payge payment protocol and I think payge plugin could be adapted easily with minor effort for installment requirements. One of our colleagues has started job, has made some changes but now he is not able to continue the work. Plugin is in some stage of development. It could be installed, initiated, sends information to bank but than stucks on banks site (it seems not sending correct or enough information). May be it needs some minor adjustments. I'll be appreciate if you can finish the job. Please see attached PHP file and installment protocol. (and for information PayGe protocol too)