If the sales invoice and purchase invoices can be linked (Many to Many relationship)
to show where the purchase has come from for this sale.
Makes it easy during year end reconciliation
That is not possible, nor is it necessary. Manager uses a perpetual moving average cost method for valuing inventory and therefore does not track relationships between purchases and sales. (Search the forum with that phrase for a recent detailed discuss.) The cost of goods sold depends on your entire previous purchase history. It is not a question of one-to-one, one-to-many, or many-to-many, but of one (or many)-to-all.