I'd probably change my approach up a little bit.

SELECT Eq.EquipID, IE.Amount as EquipAmount

FROM Equipment Eq

INNER JOIN InvoiceEquipment IE

ON Eq.EquipID = IE.EquipID

WHERE IE.Type = 'A'

AND NOT EXISTS (

SELECT 1

FROM InvoiceEquipment

ON IE.EquipID = EquipID

AND InvoiceDate > IE.InvoiceDate

)