This feature was prevented in flight 10.0.31 - its very difficult to find any documentation, nor any reason why.

Why would anyone assume this was a good idea? Almost all projects start with a customer advance

If the user instead raises an advance invoice using on-account invoice with prepayment for a new project the user still has an issue later with adjustment for the payment, since the “Prepayment adjustment” button is also disabled.

I don't understand why such an important feature is prevented.

Please restore it asap.