PXResult.Record registering as null despite values in the PXResult

Since you are using a single object being returned, you can cast prepay right to APInvoice
 

foreach(APInvoice prepay in invPrepayList)

If you did a join, you would still use var, then get the item type separately, into multiple for each type. Here is an example for getting the APInvoice out of it:

foreach(var prepay in invPrepayList)
{
    APInvoice invoice = prepay.GetItem<APInvoice>();

    //now you can use invoice.RefNbr
}