Did you know that it is possible to create a “mini-budget” module in EBA DMS without programming, which deducts the value of the linked invoices from the defined value on the contract?
Basically, the way it works is that you define an agreed contract sum, and the costing engine subtracts from this sum the amount of each invoice (contract balance) that you link to the contract, so you have an instant indication of the remaining contract sum. The corresponding balance is recorded on the contract itself, and the corresponding balance is also corrected on any invoices already linked.
The desired functionality can be implemented on any document type. See how the “Mini Budget” looks in practice: