impossible
Level 6

Sales & Customers

Instead of an Invoice, I would create a Sales Order or Quote for $50.00 + applicable tax, requesting 50% Deposit (or $25.00) These are both Non-Posting transactions that don't affect your finances.

When you receive the $25, use the Receive Payments to record it. This will create a Credit in the Customer's account.

When you fill the order, turn the Sales Order into an Invoice with the current date and apply the $25 Credit, The balance owing will be $25 + Tax on $50.00. Your Sales and Taxes are now both accurate.