How do I post a prepayment on equipment that has not been manufactured yet?

Is this where a customer pays in advance for a product. If this is, then the journal entry is: Debit Cash for the amount and Credit Unearned Revenue for the amount.

When the sale is made the journal entry will be: Debit Unearned Revenues for the amount and Credit Sales Revenues for the amount. Also you will have another journal entry for the cost of the product. Debit COGS for the cost of the product and Credit an Inventory account for the cost.