Mixed payment types

I wonder if you point me in the direction of how to acheive this:

Client owes $100
made up of $50 one off payment and $50 recurring monthly charge. So now is $100 next month and thereafter $50.

I obviously don’t want him to have to put his payment details in twice how do I achieve this?

You can create a button/pro-form in a page just for him, that’ll have an initial amount of $100 and a regular monthly one of $50.