With the additional information provided I would suggest you need two queries
the first is a 'group by' query for your invoices - it will be something like
Code:
SELECT ProjectID, Sum(InvoiceValue) as ttlInvoicevalue
FROM tblInvoices
GROUP BY ProjectID
We'll call this query 'qryInvoices'
Next you need another query which will be something like
Code:
SELECT tblProjects.ProjectID, tblProjects.ProjectValue-ttlInvoiceValue AS AvailabletoSpend
FROM tblProjects INNER JOIN qryInvoices ON tblProjecty.ProjectID=qryInvoices.ProjectID
this tells you how much funding you have left on the projects
How you apply this information depends on how you want to use it - for example are you using access to generate and invoice? if so then you would have a form to do this and you would use a variation of the query to get the figure - this could then be used to validate your input to ensure you don't enter too high a figure. Again, perhaps your invoice has one value or multiple values - you need to explain. Or perhaps you have invoices generated in another system so you need a different solution