Sorry to confuse; I thought I was commenting on a thread started by hongfeimark, that asked the question "How to create an invoice with partial payment request?"  -That is my question, but was also anticipating instruction on how the customer can make a partial payment on-line?

Fyi, mostly this would be for the purpose of a down payment. 

It is already possible for me to take a partial payment from my customer if I process the payment.  To do this I locate a box on the Receive Payment page to the right titled "Amount received" and change this value.  However, if I email my customer the invoice I want them to have this same ability to adjust the payment amount.

By and large the simplest way to work around the customer's inability to adjust the amount received is for my customer to hand over their payment information in person, or over the phone so I can process the payment myself. 

Thanks for your interest impossible, but I really just have a gripe that this function does not exist, and I know that many are waiting for it to be implemented.