How does a foreign seller get their 15% FIRPTA withholding back?

[James Bakun]

I have a foreign client who is completing a 1031 exchange, and has decided to bring extra cash to the closing table to replenish the "withheld" FIRPTA amount on the sale of the relinquished property. This will allow us to complete the 1031 exchange. With that said, we will still file the 1031 declaration form and 8288-B form with our CPA in hopes that the certificate gets back in time before closing. I just have 2 questions:

1) How can he eventually retain the "15% withheld" funds from the escrow account? Do we have to show proof to the buyer that he has settled the capital gains tax and/or provide the IRS certificate? 

2) Since he is replenishing the "withheld" sum, is it possible for us to close the replacement property on a different date? This would allow us to bypass a simultaneous closing and buy a new property within a 180-day timeframe to fulfill the 1031 exchange requirement.