Hi and welcome to JustAnswer:
The 2012 loss is allocated on a % ownership per day. Thus the original owner will receive 99.876% (100% * 364/365 + 1000/1872 * 1/365) and the new shareholder will be allocated .124%.This method is shown on page 22 of the instructions for Form 1120-S. See link below.
if the 872 shares were $1 par and sold for $200,000 how would the allocation change if $50,000 was actually paid in 2013 and $150,000 was paid in 2012 but the full stock certificate was issued in 2012, so there is a receivable due. does the fact that some was paid in 2013 impact the holdings at 12/31/12