Vtiger Specialist Need With API Skills

Looking for vtiger specialist to integrate paypal into my CRM. Need a "Add Payment" button on the invoice screen. When you select that "Add Payment" button then a payment form opens up (Payment Module Created Already) and prefills the fields with data from the invoice. Then you select "Payment Type" and if a credit card payment is selcted then a credit card dialog pops up with a list of credit cards on file for that client only. You then select a card from that pop up and select a "Charge" Button. It will then process and show success or declined message. On a success message it should mark that payment as complete and save the payment. This process has to work to automate the payment process as much as possible. Also we need the ability of changing the paypal account from the settings in the CRM settings. This budget is very small at this time because I have had two developers that has been learning on my penny. So having that said please don't bid unless you are 100% sure you can do this. I prefer that you have worked with the Vtiger application in the past. Please see images below. These just show the layout of how things should be in a round about way. I have the credit card module built, payment module, and invoice module already. You just have to tie the fields together in with the buttons and paypal api.