Only receipts dated on or prior to the GL Post date will be available to be invoiced.

Version 2002.4?  Or earlier?  Are you specifying a single PO Number in AP-C?  If you exit out of AP-C and come back in, do they magically appear?  I have seen that happen and can not figure any programmatic possible way it could be happening but it does.  I thought it had been fixed prior to 2002.4 though.

Yuck...  So whatever it is, it is not fixed in 2002.4.  OK, back to programming.

You can edit the PO Line item balance to be received to 0 rather than close the PO.  Then you can close the WO and also invoice the item in AP-C.  And presumably this is a more accurate representation of reality - if you are ready to close the work order, then clearly no more parts are coming in from whatever the service was so the balance should be zeroed out.
 
If there is a 0 balance to be received on the PO Line item, but the AP-C invoice has not yet been entered, you will still get a warning when closing the work order that there is an uninvoiced receipt but you can bypass it and close the work order anyway.  The potential risk is that if the AP Invoice contains a price change relative to the original receipt, if the Work Order is already closed, the price change will post to WIP Variance.