I create a small example for you.
Please download->unzip and install it (BF->Configuration->Package installer->select xml file->Install package). After that you will see form in Manage forms with name test32
1) I added textfield with name code, paypal button, and custom submit button.
2)in custom submit button->Advanced->Actionscript->check Custom->check Library->and select ff_validatesubmit
3) In textfield->properties->Validation->check Custom->and put:
function ff_code_validation(element, message)
if (element.value == "ABC123" || element.value=='')
return 'Please enter a valid discount code';
} // ff_code_validation
In my example discount code is ABC123 replace it with yours.
code is name of textfield (not title)
3) in form->Advanced->Visibility rules I added this:
turn on element paypal if code isnot ABC123
turn off element submit if code isnot ABC123
turn on element submit if code is ABC123
turn off element paypal if code is ABC123
ABC123 replace with your discount code.
The example works on this way:
if textfield code is empty or not ABC123 paypal button is showed.
If textfield code has code ABC123 submit button is here, paypal is off
I hope I explained it well
If you need more help, please let me know