Applying credit to a proposal

Hello The Enlove Creative! Michal from HoneyBook here.

Let me know if I understand your question correctly:
You would like to send a proposal, including (I assume) a service selection component to select their desired product, with an invoice, to a client that has placed a deposit in the past, and you would like to display that deposit in the form of a discount in the invoice. Is this correct?

If this is right, then the limitation of the $100 discount is not arbitrary, but probably a result of your service selection and invoice structure, and I’ll explain.
The maximum fixed amount that can be given as a discount depends on this calculation: the total price of services already in your invoice + the lowest price between all mandatory selectable services in your file. 
For example, if your invoice contains no pre-selected items (= $0) and the cheapest service a client can select is $75, then the maximum discount you can give is $75.
Another example: your invoice contains a mandatory $50 payment for a 1:1 call, and a client can select one of three other services, the cheapest of which is $150, so the maximum discount can be $200.

So you are probably bound to that $100 discount because your cheapest selectable service costs $100. 

There are a couple of options I can think of to tackle this situation:

1. In the invoice, add an item for “a downpayment” (or something similar) in the amount that was actually given as a downpayment (in your example, it’s $300), and then you can add a discount on that amount.
2. Send a proposal without the invoice/payment component and set the file to create a draft invoice once the client submits their selection. In that draft, you can edit a discount based on the amount they’ve already paid and send them the invoice separately.
   It is this section in the file’s settings menu (as long as the file contains services):

Let me know what you think!