If I wanted to provide a useable "allowance" in advance for customers which may or may not be used, what would be the best way to handle this? This would be a value for the year from which this can be drawn down until it is used up, then the customer will pay thereafter? Only thing I can think of is to provide a credit balance and manually pair off with Journals but hoping for something a little more elegant

What is the expected GL impact of that "allowance"?

Ultimately, nothing if not used, if used it will credit debtors and debit an allowance GL

A few way to think about: a) a custom Customer field for the allowance and then have a saved search to track the progress of the usage. b) some kind of line level figure which tracks the ongoing balance and once zero, no longer reduced the $ c) An after submit script that creates the journal and updates the customer invoice with a credit line (until allowance is consumed. d) maybe marketing would like to make use of the allowance usage and show the customer.