A partially paid sale (a LayBy) is handled as a positive entry in the LayBy Liability Account with no tax applied.
There is an implicit expectation that the LayBy will be fully paid at a later date when the actual sale will be recorded including the TAX for the sale.
$1,100 Sale paid $400
Calculated Summary:
$400 recorded in the LayBy Liability Account.
$200 added to the $1,100 LayBy
Calculated Summary:
$200 added to the LayBy Account
$500 paid that convert the LayBy into a Sale
Calculated Summary: 
$600 credited from the LayBy account, $500 payment applied and a $1,100 sale (including TAX) is recorded.