A line item cannot be receipted more than once unless there is more than one item ordered. If it's receipted multiple times, it will read as over receipted and post a 0.00 amount. Once the line item is fully receipted the full amount will post regardless of changes made to the Unit cost.

To workaround this, either increase the Quantity ordered and enter multiple receipts, or create separate line items corresponding the amount of receipts.

The other option would be to fully receipt the line item and create an invoice for the full amount and schedule the payments.